whats the acceptable bandwidth for an ip phone conversation ?
The bandwidth requirements vary depending on what Codec is being used. Here is some related info. The first doc has a good link to the TAC Voice Bandwidth Calculator which is very helpful;
Voice Over IP - Per Call Bandwidth Consumption
Q. What are the voice bandwidth requirements for different codecs?
A. Voice bandwidth requirements are:
G.711 - 106 Kbps (50 pps, 200 byte long packets)
G.729 - 44 Kbps (34 pps, 60 byte long packets) (default)
G.723 - 30 Kbps (34 pps, 64 byte long packets)
Hope this helps!