When you make a VoIP call it will send and receive data packets over your ADSL link. The amount of bandwidth it uses really depends on two things; the type of codec you are using and the number of parties in a call.
Codecs generally have the greatest effect on the amount of bandwidth in use. The two most popular codecs, which are also supported by Wi-Manx are G.711 and G.729a. The raw bitrate on G.711 is 64Kbps while on G.729a it’s 8Kbps. However, in the real world when you take into account overhead (multiple levels of encapsulation) the bandwidth usage is like;
If you use video with the various codecs (MPEG4 etc) then bandwidth requirements will increase. Using G.729a 4-5 conversations are possible on a standard ADSL line, using G.711 we generally recommend 1 or 2 at the most.