Hiya,
My BX2000ac arrived 2 days ago, and everything I've tried (without reading the minimal documents) has worked.
I've configured VoIPtalk and DrayTel SIP trunks for inbound and outbound traffic.
Dialplan is working for routing mobiles via one ITSP and landlines via another. GSM Gateway is on order for 3rd trunk!
2 VoIP handsets configured as extensions and a group.
The PSTN trunk is working, but not the way I'd like it to
Auto Attendant is answering, but I've not customised any of that yet as this interface is new to me (I've done SIP and VXML before).
This was installed as an upgrade to a 2860 and all VPN, VLAN, DDNS and Wifi functionality remains functional. Only noticeable loss is the AP Management stuff and the WAN2 Wifi failover (not documented).
Early days, but no show stoppers so far.
Chat soon.

Good to hear - thanks for the update.

An update for all:
I have spend some time with Draytek support and done some of my own tests.
The PBX audio etc works fine when used with both WAN 1 (VDSL line ) and WAN 2 (via Bt openread modem). So no issues there (so far).
There is still no audio when using WAN3 or WAN4 via a USB 3G/4G modem from the supported list.
I have tried 2 different types of supported modems, 3G using PPP connection and 4G using DHCP connection.
I have a SIPGATE and a DRAYTEL SIP trunk registered but no audio on either. I also have set up a directly registered trunk from the handset to SIPGATE (this works fine with a STUN server enabled on the handset).
Tried the following options and combinations thereof:
ALG enabled and disabled (using Telnet commands).
Router STUN server enabled and disabled.
Opened all the relevant Voip ports.
Disabled the firewall.
Internet access etc is fine.
Can't use Wireshark to trace the problem as the router will not allow mirroing WAN3 or WAN 4 to a LAN port.

I've really spent a lot of time on this, but I'm not an expert in this field. I am pretty confident it's a software issue rather than a NAT issue. I believe the NAT works the same whichever WAN port I use?? (anyone?).
I would be grateful if anyone has any other suggestions or work arounds. My business runs on 3G/4G connection as VDSL is not available here. In the usual Openreach customer service, the FTTC box is at the end of the road and working. I have a cable and JB already in the unit, with a phone line that was previously working. They just can't be bothered to patch across the 30 or so businesses on the estate to the cabinet. If I can get this working over 4G I shall never rent a BT landline again. The neighbouring businesses are eagerly waiting for me to get this working before they consider buying one as they are in the same boat with their ADSL speeds.

any help appreciated.

Update:

Latest firmware on BX2000 now seems to work with 3G/4G dongles. Well at least on the Three network it does and it uses Carrier Grade NAT. So things are looking good!.

Check your registration timeout values to ensure it doesn't drop after a few minutes. Mine is set at 30 seconds at the moment to get it to work.