there is a list of them, and which one is for what here: http://wiki.howardforums.com/index.php/T-Mobile_APNs

I believe most people are supposed to be using epc. right now, aside from sidekick and total internet.

VOIP works best with good QoS, and enough bandwidth, are you getting 3G all the time? There could be a number of different things that would cause a drop out on calls like that, but i really doubt its the APN u are using.