Once a user made a call and Asterisk began to display a lot of warnings, and there was also no sound:
WARNING[C-000000c7]: chan_sip.c:7296 sip_write: Asked to transmit frame type ulaw, while native formats is (g729) read/write = ulaw/ulaw
I looked at the installed codecs and it turned out that it was not installed:
asterisk -rvv core show translation exit
Since the problem needed to be solved urgently, I did not install the codec, but simply commented it out so that it would not be used:
[ukrtelecom1] ... disallow=all allow=alaw allow=ulaw ;allow=g729 ...
Rebooted asterisk to apply the changes:
service asterisk restart
After that, the problem was resolved.