Hi Keith,
Apologies for the delay, this project was unfortunately on the back burner for a bit there.
The problem seems to have shifted a bit - I'm nearly there, but I'm running into an issue. For some reason, my calls from one MS are being automatically rejected by the second MS, which says "Incoming call rejected while busy". I've included pastebins for the various configs and debug outputs below.
extensions.conf (Asterisk): https://pastebin.com/HXQb3Hux pjsip.conf (Asterisk): https://pastebin.com/S7pyq0YH mobile output (Asterisk): https://pastebin.com/Z63xkvLW
As always, thanks for any help you're able to give, it really is appreciated!