ChanSIP (or referred as SIP in VitalPBX) is deprecated and will soon be removed from Asterisk.
So the real answer here is: Switch to PJSIP.
Unless they use a tel:uri (to which they shouldn’t). Then there’s nothing they need to support and should work without them stating that they need to support it.
I suggest you try to set up a PJSIP Trunk and report back any issues you have. (Remember that ChanSIP and PJSIP runs on different ports)
Happened again with SIP.
“sip show registry” says “No Authentication” and then it will never try again. But actually just the Internet was down for a while. VitalPBX does not show that state.