Just guessing, when the mobile has a VoLTE connection, Asterisk rejects the call because of a codec mismatch.
If you can cause the trouble by calling from a specific mobile, at the Asterisk command prompt type pjsip set logger on
(or sip set debug on if you are still using chan_sip)
paste the Asterisk log for a failed call at pastebin.freepbx.org and post the link here.
If you cannot cause the trouble at will, see
for various ways to capture traffic continuously until trouble is reported.