I need some technical support with VigorBX 2000 and Vtech VSP725A

I am unable to get the two configured to work together.

If I configure the Vtech VSP725A stand alone I am able to make and receive calls via our SIP services (So I guess that means I have the correct details).

If I configure the Vigor as a PBX and the connection to the Sip service I can receive but not make calls via the Draytek soft phone and I can get the Vtech configured as an extention to ring but I can not pick up in coming calls on the Voip line Vtech.

I can make calls between the Vtech and the soft phone correctly.

I have been through the setup guide on the draktek site for the Vigor BX2000 and the VSP725A and I think I have it exactly as shown.

I have what looks like the same issue with a Grandstream GXP 2010 deskphone.
Phone registers fine direct to SIP provider (SIpgate). But when used as an extension on the BX2000 PBX, it rings in and out fine, but no audio either direction on the call.
On the internal side, there is audio to voicemail system, so I assume the extension side registration and settings are OK.
On the external calls there is no audio to either the voicemail or the auto attendant when dialling the main number from outside.

Is this the same as you are seeing?
I've tried every setting possible I can think of and its an expensive brick at the moment.