DNS and STUN are the routine causes of such a thing. You haven’t provided any Asterisk side logging such as console output or stated the version in use, so that’s the extent of what anyone can say.
We are ussing Asterisk 18.9.0. No STUN configured. About DNS, it’s a local asterisk server, the problem is the communication between users and asterisk server, Is DNS needed for local communication too?
DNS is used for various things. I see you’re using chan_sip, I’m going to bow out of this thread and someone else may chime in. I’ve done my time with chan_sip.