CloudPBX uses the G722(64) and G711u codecs, which will use 134 kbps per call (67.2 kbps in each direction).
You'll find that the 'upload/upstream' side of the last-mile connection will often be the limiting factor, unless fibre is installed.
As a rule of thumb, any uplink that provides '1M upload speed', which is the most common for ADSL links, will normally permit 12 concurrent calls. The calculation shows that it can be as high as 15, but we just assume 12 to be a little conservative.
In addition, the actual bandwidth capability of any location will vary - and is almost never what is advertised as the maximum, due to location constraints.
The selection of ISPs is something that can be tricky -- but is extremely important.
For good quality calls, this is probably the most important decision in the process - low jitter is the key here, with no packet loss of course. Latency is important, but changes in latency (jitter) is far more important. You ideally want to see this as less than 10ms. Sound quality will be impacted once jitter reaches 16ms+, and more than 30ms jitter will have serious impacts on call quality.
It might be best to run MTR tests from several last-mile ISP routers to the CloudPBX SBC that your phones would register to, in order to get an idea of how different ISPs will perform.
The endpoint public IP does not need to be static, DHCP is just fine. Static just makes it easier for you to manage the phones remotely. What is important is that your own router be used, rather than whatever default router is provided by the ISP - it is best for the modem to be installed in 'bridged' mode.
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