Has anyone had the PBX side of the new BX2000 working yet? I'm having so much trouble -
No audio to from external calls. To either extension or voicemail /auto attendant.
Voicemail password not being accepted.
SIP trunks may decide to register if it feels like it. Sometimes after 10 minutes.
No internal Radius server despite the documentation saying it has.

Getting very frustrated here. Tried everything. Any assistance would help.

Using a 4G dongle on USB#2 / WAN4. But phones register directly to SIP provider without issue.

I recommend to contact support for these issues - http://www.draytek.co.uk/support/techquery

In my experience, the BX2000 worked quite well, though I haven't personally tried Grandstream handsets with it.

Have you tried the DrayTek Softphone?
It may be worth testing that alongside the other phones to see if it behaves differently.
You can get that from here: http://www.draytek.co.uk/support/downloads