Assuming G711 codec, 20ms packetization and no silence detection
the amount of bandwidth needed for simplex stream is in the order of 100kbps (a bit less actually).
In reality, in a typical calls there can be on the average about 60+% of silence (or more): for example,
when the other person talks, you normally don't talk. And also even when you talk, you don't
talk 100% of the time. So if silence is taken into account, the bandwidth for a simplex stream can
be even lower by quite a bit. So even though 100Kbps is an overestimate you can probably use
it as a nearly worst case guideline.
Other codecs supported by Obi should be using even lower bandwidth.
Not sure why the report you point to shows such an impact. But I'm guessing that something
else is causing it and not actual bandwidth usage of the voip stream.
L.