Hmm...
Well UI is the user interface... I am assuming you have not changed yours.
Example: my phone had the verizon user interface and I hacked it and put the motorola user interface in so I can skin it.
Typical ring time is 30 seconds.
I would first go to
HowardForums: Your Mobile Phone Community & Resource
Register (its free)
Then post this question in this section
HowardForums: Your Mobile Phone Community & Resource - AT&T Regular Phones
It's the AT&T section.. they are the best people on the net for all things cell phones.
See what they say...
If you don't want to do that... bring your phone to the store and tell them the problem.
Check also to see if they have a working model on display... call the working model with another working model and see how long it rings for. That way you can see if it's the same or different from yours.
15 seconds or less is crazy... that's not enough time to get it from your pocket and see who is calling!
Good luck.