When setting up SIP-accounts from some service providers or registering with PBX, whose settings we do not have in the list of server templates you may need to manually specify the list of codecs used and their priority.
The digits after / are the sampling rate. If not specified, the standard 8kHz frequency is used. The exception is the Opus codec: it operates at 48 kHz.
mulaw is the telephony standard for the USA, alaw for Europe. /16000 are their high quality versions.
Opus is a modern, high-quality codec used by Telegram for calls. Opus is resistant to network loss.
L16 and variations via / are raw codecs. L16 is not loss tolerant and requires a good quality connection, so works best when connected directly to a WATS.
Example of setting a codec set
Values are separated by commas, no spaces, case insensitive.