Bandwidth required for VoIP is standard from the gateway side meaning that you decide on either G.729 (8kb) or G.723 (6.3kb). These are standard codecs and the bandwidth used for each is standard for any device. So if you use G.729, the payload size will be 8kb in each direction, however, you need to add the IP header information to the payload to complete the packet. This part varies slightly based on your router and the network. Typically it is between 10kb & 16kb (yes the header is actually bigger than the payload). So for G.729 the average bandwidth needed is around 18kb or so in each direction per call.