It depends.

Audio codecs

Most commonly used are:

G711

PSTN quality codec which is supported in 99% of all devices/providers (but often disabled to save traffic).

It has best quality but highest bandwidth (64kbps + ~20 kbps overhead ~= 84kbps to one direction).

It comes in 2 flavours - G711u (ulaw) - used in USA, and G711a (alaw) - used mostly in Europe and other countries.

Little worse quality but is often used because it saves traffic (~8kbps + ~20kbps overhead ~= 28kbps to one direction).

Read manual for your device to check which codec is supported on your device.

H263/H264 are most popular.

Make sure same codec is chosen between devices you are trying to communicate.

Make sure you have always selected audio codec! Otherwise you will not be able to place calls!