Channel N/M, span X got hangup, cause -1

I am trying to get Call Forwarding working. Basically I have set up Call Forwarding through the Asterisk GUI. I have set up one extension to forward to a mobile number. If the extension is phoned internally it works (i.e. I get forwarded to the mobile). However if I dial the extension externally I get an immediate hang-up.

The verbose log from asterisk is here:

The relevant sections of the dialplan are here:

If anybody can tell me what is going wrong here that would be great. I have found a few other articles on line that suggest that you have to set the cid to be a valid one for our group (which I have done).

We’re using SIP phones internally and we have a channels of a PRI externally. The system is built using Asterisk