Alright, I'm going to provide as much information as I can and possibly try to ease some of the people worried about their spouses/significant others cheating due to the data usage on their bill that they deny.  I ended up writing a novel here, so I apologize for the hefty and possible unnecessary load of information.

*GOOD TO KNOW*  -although this is becoming a much smaller issue, when you are outside of your "coverage area" using by chance, a different network service you MAY possibly be charge for roaming data usage. ie, your service location is your immediate area. Different states and regions often have different tax fees. Most wireless plans now however have a nationwide coverage plan where you won't inflict these charges to such a harsh degree.

First of all, data usage applies for EVERYTHING, except (respectively) voice call, and sms messages. Data usage can be due to:

-MMS text messages (mutilmedia text messages, ie. picture/audio in a text message, or a text that exceeds its character limit) (

***IMPORTANT NOTE*** ! This is a BIG ONE! Even if YOU (as well as the sender/recipient) have an unlimited text messaging plan, MMS messages are charged as DATA as they travel through a different type of frequency.

-Internet/Brower surfing

a. For people with older models and even non-smartphones, with a wap.cingular browser or mediaNET brower, even opening it up sends data. Even on accident, opening it and canceling it immediately, STILL sends data, if only 5/kb. Most with these phones do not have a dataplan at all, (defalt pay-per-use) and sadly this definiately adds up. especially if you do not exit/close the browser.

b. For smartphones, iphones, all those with a built in OS brower sends data for every page you view. Often, pressing home 'exits' the browser, but it is still there, sitting on the page you last viewed. This will send data even when you are not actively viewing it. These phones REQUIRE a dataplan for several reasons, this being one of them.

-Downloads

This includes, but isn't limited to:

Ringtones

Apps

MP3s

Videos

Pictures (JPEGs, etc)

Email Attachments (viewing email as well)

FOR GO-PHONE, NON-SMARTPHONE, and OLDER SMARTPHONE MODELS (with no OS)

Most people with these phones do not have a dataplan at all, and/or a very low limit one. I strongly recommend for anyone who actively uses the mediaNET brower and AT&T apps that come with the phone such as AT&T radio/tv and or location based apps ie, AT&T Navigator/FamilyMaps to get a unlimited dataplan. For eligible phones such as these, its very cheap and can easily avoid any data usage. Be sure to check this regularly, as many phones who were considered 'smartphones' fall into this catergory.

Apps such as Weather, Stocks, News, Email, and others on these phones CONTINUOUSLY & ACTIVELY send out data to update information, without the phone being used.

!!!! THIS CAN EASILY EXPLAIN UNKNOWN DATA USAGE IN THE MIDDLE OF THE NIGHT. !!!!!

AT&T has recently changed their DATA PLANS to limited set usage instead of unlimited. For people with older smartphones that required a dataplan, they are allowed to keep this service with the select phone. (LUCKY) if you upgrade, as in from an iphone3G to iphone4, or to any newer smartphone (that does not fall into the catergory of the phones I described above) you WILL LOSE THIS OPTION INDEFINITELY. (as well as starting a new plan, and even editing the features for your plan)

MANY PEOPLE who do not PROPERLY look at their bills and manage their features did not accordingly adjust their data usage with this change. AT&T has a very useful tool on their website that shows you what actions can use data and how much, respectively.  

NOW FOR THE BIG DATA DEAL BREAKER!

FOR SMARTPHONES, especially touch interface phones, i-PHONES & ANDROID PHONES! 

*********SYNC*******

Your smartphone syncs. ALL. THE. TIME. At all hours of the night. Whether you use the phone or not. Several apps that come built into the phone are automatically set to update, send data, and sync whenever it wants. Some apps, (as I have found on my new HTC Inspire 4G, CANNOT turn off syncing COMPLETELY, like the Stocks app which I don't even use, but has to sync every 4 hours at the least) Even the WEATHER app/widget constantly syncs and updates unless specifically set not to.

Social Media APPS, such as Facebook, Twitter, Gmail, these are CONSTANTLY 24/7 sending out and recieving data, unless you specifically limit them to only when you use/open them, or hour intervals like every two/four/ten hours, or even once a day. This is a deal with many teenagers. They often use the social widgets which always send data as to show status updates and messages.

Instant Messaging clients & apps such as AIM, YAHOO MESSENGER, AND MSN MESSENGER(windows live) also use data (though many phones have a client built into the phone that can be charged as SMS(text messages) though this is dying out sadly. Many teens leave this client signed on.

EMAIL!!!!!!! ATTENTION WORRIED WIFES & SPOUSES******

If your spouse uses your phone for WORK & BUSINESS, or email AT ALL, they will often have it set to automatically update. This means ALL SPAM & JUNK EMAIL will SHOW AS DATA CHARGES ON YOUR BILL, at ODD HOURS OF THE NIGHT IN VARYING DATA SIZES!

For uneases spouses, you CAN look at your COMPLETE bill to see WHAT data is being used. It can tell you if it is a text message or data transfer (internet, email)

There is no way to tell the difference from the data transfers on the bill (whether they are email,browsing,apps,syncing). you will have to look at the phone if you are still worried but keep in mind if you or anyone on your plan has a newer smartphone, they send data continuously, often without the user's knowledge. Keep that in mind before confronting them about suspected cheating or your child doing things they shouldn't.

I used to use an HTC Aria, android smartphone with a 2GB/month data plan. I never came close to this limit even with never changing my syncing options and always running social apps. When I changed to the HTC Inspire 4G, I continued to have the 2GB/month, and was surprised when my data SKYROCKETED. Turns out all phones capabilties send and recieve different amounts of data. I was stunned that my phone was constantly eating up data so fast. Any 4G phone will use more data even running the same things. I don't know why, especially since they are still (despite claims otherwise) using the same 3G network. After setting the syncing options to the minimum, my data usage lowered impressively.

My tips for lowering data usage:

1. Manage all linked accounts, such as Gmail, (google) Facebook, Twitter, and auto-sync apps (many come with the phone such as Stocks, Weather, News), though some may be downloaded. Most phones have this folder in settings. When unchecking the "Sync automatically" box, You can modify when and how often your linked accounts and apps sync to update their information. I have all of mine set to update once a day, or if I open or actively use the app. Also uncheck boxes that state any apps can sync, recieve, and send background data at any time. This applies to all other apps, like games or any you download from your app store.

2. Make sure your GPS  is turned OFF, when not being used. Many many people I know forget about this and/or don't even realize when it's on, and didn't realize how much data it incurred as well as why their phone died so often. A good indicator of it being on is that it's a MAJOR battery drainer. When using location serviced apps like maps that work better with GPS satalites, make sure you have it set to ASK everytime it wants you to turn on GPS satalite access. And be SURE to turn it off when you are done using it. (not sure of others, but I have to manually turn mine off.

3. Utilize Wi-Fi when it is available. This is the best way to avoid data, granted you stay connected to Wi-Fi. Many people have trouble keeping an active Wi-Fi connection even if they are close to their router.

** Most people also have the option to turn off the mobile network when your phone is idle or asleep, (not actively being used) so that nothing can sync. This means NO data can be sent, synced, or received (apps, email, internet, & mms txt messages) until you wake up the phone.  (which usually, unless manually set otherwise, toggles mobile network back on)

You can still receive calls & sms text messages without the mobile network enabled.  It will promptly resume when you do use the phone again unless you have it set where you must manually enable it. This helps people limit unknown data usages at odd hours as well as save battery life.

4. Regularly clear the cache from your used apps when they get too high. this helps save battery, as well as keeps the app running smoothly & preventing errors.

**With internet browsing, make sure to fully manage your browsing settings, like cookies, display options, and cache limit. (I recommend 2MB cache limit for webkit 3.1 android browsers)

5. Manage your running apps & downloaded apps (Mostly the apps that sync/update very often, like social apps)

Sadly alot of apps don't like to completely "close" and will run silently in the background. Most of the time the phone can appropriate these and make sure they do not slow down your CPU, though some may be data bandits. If you have your mobile network always on, they will, slowly, eat up your data even when you think you exited them. One thing especially is the internet browser. Pressing home will most likely keep it running in the background. I myself have yet to find out how to properly exit the internet browser.

 I suggest regularly checking your installed applications in your settings, and force closing any you do not want/can refrain from running. Keep in mind, this will delete any unsaved and temporary data within the app, so be sure you've fully completed using it before force closing or killing the task. You also have the option to clear data. It will warn you upon doing so that it will erase all user settings and preferences. Usually the amount of memory this takes up is so small it wouldn't make a difference.

You can find several task killer apps in your app store for easier management, (although I do not recommend them, it is a always-running app, eats battery, lowers CPU function and occationally stops apps unneccessarily, which can corrupt them, error, or delete sensitive info). Usually the smartphone does an acceptable job of managing this itself, however in the event of an app going haywire, force close can isolate the issue. If you find yourself not needing an app you downloaded, just uninstall it to prevent unnecessary data usage from syncing or updating.

**(Note: some functions and applications in your phone that show up on the application screen NEED to run, usually built into your phone. for example, the touch interface, or display function, several system tools like your dialer, call storage, contact storage, audio settings, even the android system. Obviously it wouldn't be in your best interest to force close these, although in a few cases even doing so, it seems they will immediately start back up.  I can count on one hand how many times I've done this to a select few, but I don't know the damage this can do to your phone if you do it continously to any or many of these. It's relatively easy (at least on androids) to tell which ones are best left alone. For people who may immediately turn trigger happy on managing applications to reduce data and memory, they may just go down the list and force close these in a routine. Try to refrain, like I said, I don't know the possible

damage it can do and don't recommend any of you trying to find out.  **

**(Related Note: Some apps and their services, although not particularly esstential to your phone's immediate function (as well as not even included in your serviceplan, like txt/sms) can be force closed though they do run continuously without your input. An example of this is: sms messing service. When you force close these, usually they will also immediately start back up. This is a phone capability. It means your phone has the ability to recieve and send texts built in and you can't permanently turn this ability off. Again, I don't know the harm or effect this can have on your phone if you force close or stop this repeatedly.

6. Keeping track of your data usage. Like stated before, AT&T now provides many useful tools to estimate and track  your approximate data usage. The Data Usage Calculator, (although I don't seem to find this too accurate), and a service text message. Simply Dial #3282* ( #DATA* ) on your phone, and it will send you a free txt message showing approxiate data usage beginning from your last billing period. This is my best friend. You can also check usage for you and all lines in your plan with an online management account on the AT&T website. Also very helpful.

** They do have an add on feature to set limits to your data usage in select forms, with AT&T Smart Limits. I myself haven't tried this, but for parents with familyplans who want to limit their teens data usage so they don't over use, this may be a good choice, and can even help with your own data usage if you find yourself needed more strict limits. You can also use the free parental controls for limited browsing content, assisting in lowering data usage as it prevents broader mobile web (and mature chatting sites, as well as buying downloads!) This is good for non-smartphone & non dataplan phones, and smartphones with required dataplans as well, though the data limit with Smart Limits may be a more effective option.

Knowing how much data you normally use with your respectedly phone (as the amounts seem to vary based on phone's capabilities) will help you choose a dataplan that is appropriate for you.

 SO, TO CLARIFY - understand that almost everything your phone does requires data. Don't jump on the notion that your spouse is cheating.

 Thanks for reading, hope I helped. Almost sticky/recommended thread material, I hope.