I found the solution by myself.
In the dialling plans, I had any number beginning 1222 set to route over SIPPhone. This was bringing up an engaged tone constantly.
What I then did was to go into the same dialling plan to edit it in advanced mode. I then switched off the ENUM look-up and this corrected the issue with the conference rooms and SIPPhone service numbers (I did the same for all numbers starting 1747).
As an additional note, UK Freefone numbers were being routed incorrectly when I had set them to go through SIPGate.co.uk, again, disabling the ENUM look-up corrected this and the UK Freefone calls were routed correctly through SIPGate.
I hope this helps a little in return for the help I have received and asked for so far!
