I have experimented with this problem and found the following repeatable behaviors.  Equipment (Microcell 3g, iPhone 3GS, AtlanticBB cable in Western PA)

1.  The delay seems to occur whenever I return home and enter the MicroCell range.

2.  The first call to any number does not connect and ends in a call failed message.

3.  Immediately repeating the call, it will go through.

This behavior is very repeatable and seems like the sort of thing ATT would be able to act on.