Did you use an international calling card? You could be charged data for calls.

Let me just say that I have a 3GS and very similar thing happened on me a month ago even before iOS 5 hit. All of a sudden I used over 200 MB of data while I averaged 160 in the past 12 months.

I dug into my phone bill to find a clue and found that one phone number I called was pretty suspicious. It was a calling card (something like 10c per minute to call abroad) that I have been using for years. I found that I was charged data whenever I called access number for this card. One of the calls I made was more than 15 minutes long resulting in 90 MB of data usage. I suspect the phone card actually uses IP phone technology and somehow that messed up in the ATT system causing it to think it's a data connection. I am attaching the phone records below and you can see the caller ID to this number is listed as "PSDN MAIN".

I have since changed to call a different access number and the problem hasn't occurred. Needless to say ATT reps on the phone were not so helpful even when I told them of my investigation. Still I am happy I got refunded my data usage charge.

Here is the log. Note the time coincidence on both. Warning: do not call that evil number with your iPhone.

Voice:

Date         Time         Call To         Number Called         Rate Period         Feature         Minutes

09/24         08:45 AM         PSDN MAIN         626-389-0035         NW                   2

09/24         08:18 AM         PSDN MAIN         626-389-0035         NW                   16

09/23         07:36 AM         PSDN MAIN         626-389-0035         DT                   1

09/19         07:19 AM         PSDN MAIN         626-389-0035         DT                   10

09/16         07:54 PM         PSDN MAIN         626-389-0035         DT                   16

09/16         08:18 AM         PSDN MAIN         626-389-0035         DT                   5

09/13         07:12 AM         PSDN MAIN         626-389-0035         DT                   15

09/11         06:02 PM         PSDN MAIN         626-389-0035         NW                   2

09/09         08:15 PM         PSDN MAIN         626-389-0035         DT                   8

09/09         08:14 PM         PSDN MAIN         626-389-0035         DT                   1

09/09         07:54 PM         PSDN MAIN         626-389-0035         DT                   20

09/09         07:53 PM         PSDN MAIN         626-389-0035         DT                   1

09/09         07:51 PM         PSDN MAIN         626-389-0035         DT                   1

09/09         07:49 AM         PSDN MAIN         626-389-0035         DT                   14

09/06         06:29 PM         PSDN MAIN         626-389-0035         DT                   1

09/02         07:40 PM         PSDN MAIN         626-389-0035         DT                   16

09/02         07:40 PM         PSDN MAIN         626-389-0035         DT                   1

09/02         07:21 PM         PSDN MAIN         626-389-0035         DT                   1

Data:

09/24         08:18 AM         phone         Internet/MEdia Net         Sent         91671KB

09/13         07:12 AM         phone         Internet/MEdia Net         Sent         1906KB

09/11         06:02 PM         phone         Internet/MEdia Net         Sent         54KB