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Bandwidth used by VoIP calls depends on the Media Codec that is being used for the VoIP session, traditional phones lines user the G711 or PCM encoding which requires 64kbps of bandwidth. G711 can also be used in VoIP but it isn’t bandwidth efficient . To be used with VoIP more resilient and light weight codecs have been developed which provide Voice quality comparable to a normal phone while using very less bandwidth and resources

Commonly used VoIP codec’s include

Codec Bandwidth Used

G711 64kbps
G723
G729 8kbps
G726 16/24/32/40kbps
GSM 13 kbps
iLBC 13-15kbps

Each of these VoIP codecs not only differ in the bandwidth that they utilize but are in fact totally different algorithms for the coding of VoIP voice packets varying in things like CPU processing load, Silence suppression, VAD etc. The quality of voice provided by these also differs.

So in short, we cannot generalize that how much bandwidth will be utilized if we use VoIP for making calls as it is tied with the codec being used for that particular session