Setting number of rings before rolling to Voice Mail

Does anyone know how to set the number of rings before a call goes to Voice Mail? My phone (or AT&T) seems to be variably set to roll over at 2-3 rings, which is just too short.

I haven't seen a way to do this in the settings, and I've looked at the AT&T web site and don't see it either.

This is unrelated to the situation that occurs when one is checking e-mail or using the web via EDGE.

Thanks!

Chris