That's interesting, and rather good news. It's possible that the SIP / UDP session is being closed after 15 minutes of being idle by the 4G modem it's going through. Have you tried reducing the registration timer / Expiry time on the SIP trunk to 5-10 minutes?

Admin, thanks for your feedback. I have reduced down to 30 seconds and no luck. Still deregisters trunk after about 5 minutes. Then reregisters again after a few more. Ill keep an eye on it.
Could you advise if there is a log/ technique I can look at on WAN 4 to isolate any SIP traffic and see whats going on?

Well, packet capture wouldn't be an option, I think getting the output of "voip debug show -w" when it's working and when it's not working should give some indication of what's happening.
It may also be worth getting a PC connected directly to the WAN4 USB modem and see how a SIP client with those trunk details behaves with that, to help narrow down whether the issue is with the PBX or the modem.

Thanks Admin3
I've been using a SIP deskphone registered directly (bypassing PBX) with my SIP provider for around 3 years over 3G/4G. No issues with this or previous Draytek routers.

I have monitored it for a day now and while the trunk did register, deregister, register etc for a while. It does not do it now, and has remained registered as far as I can tell for about 24 hours or so. With a timeout value set at 30 seconds.
I don't know if there is a training mode or something on the router trying to detect the timeout value, but it seems unlikely.

I'll keep an eye on it for a few days and post up the results. It's looking promising at the moment, all functions working.

Thanks for help so far.

Been a month now, and all works fine with the SIP registration and audio over 3G/4G dongle on Three. No degistration etc etc.

The voicemailbox could do with work though. I deleted a voicemail and its still there, although the voicemail indicator goes off. Still there a month later and can't delete it. Some messages you can, some you can't.