Daniel-Constantin Mierla writes:
The mysql client library is stuck, waiting for a poll event to happen. It is the insert done for accounting on SIP response.
It is hard to say what is the reason, maybe you can install the debug symbols for lib mysql client library and the the backtrace shows details of what happens inside the library. The mysql modules was not significantly touched for a long time, so might be something specific for lib mysql (mariadb) client.
I'll try if the problem persists with newer libmariadb3 in Debian Buster and/or without cluster.
-- Juha