How Much Internet Bandwidth Will I Need / Use?

Typically an ATA (VoIP adapter) would use G.711 (64 Kpbs uncompressed).  That is per direction and based on who is talking.  If both parties talk at the same time your usage is 128 Kbps + overhead... we use 160 Kbps as a top end max possible.
On a softphone, we recommend using G.729, which reduces bandwidth to 96 Kbps (again, top end).
More detail on the various supported codec can be found here:
Basically, if you were using your phone 24 hours a day, 7 days a week, normal conversations, G.711...
You would use approx 30 GB per month. But since that usage will not happen, you still know your possible top end possible.
With that said, you can generally calculate your usage based on 1 MB per minute or 60 MB per hour usage (with G.711).

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