I think his problem is not with the ITSP at all, but that he is seeing errors as a result of his phones trying to register before he has configured them and, because the message says REGISTER, but he doesn’t understand that it is about an incoming registration, he, wrongly thinks that the outgoing registration to the ITSP is failing.
As such any direct attack on the error messages is just hiding the symptom.
The softphones (Zoiper and Linophone) both register with VoIP.ms and 3CX). I also used the Hello, World sample from Asterisk and was able to register to my server, dial 100 and hear the RVA.
The problem is then with my pjsip.conf settings, not the softphones.