SIP call not going to voicemail


I have setup a SIP trunk with Connexin which is working fine for both incoming and outgoing calls. I have setup the incoming call destination to an extension but when the call gets to the point of transferring to voicemail I hear “This account number is not valid”.

I have tried setting the incoming destination straight to the voicemail box and this works perfectly.

Does anybody know why this might be happening?

Many thanks