There are many factors involved to provide you a specific answer. Such as what CODEC do you prefer, G.711, G.723, etc. What type of QoS are you looking to provide. I can tell you from experience at a VoIP Carrier, that we could pass approximately 96 voice calls through a T1 to Asia, at a an estimated 3.5 MOS. So you are looking at 16k/sec per call. Not Tier1 quality but pretty close.