with experience, on the other hand, many will give completely different numbers for actual use. remember also that this depends on the type and reliability of the bandwidth (broadband, DSL, T1, Fiber, Shared Fiber).
I generally go with (per call): 100k for ulaw, 33k for gsm and 20k for g729.
that is not for "ability to make a call", but for "to hold clear phone conversations". If you build it with those numbers, you will likely be able to maintain good connection/call quality.
you will find many other numbers, and also technical numbers. that's just MY experience.
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