Almost I think! Thanks for the help so far.
The thing is that the external number rings at a completely different phone system, not related to the Mitel in any way. We just have one Mitel phone at this remote office, no Mitel system, just one phone that connects across the VPN. So the long full external number does not relate in any way to the extension number. The reason for putting the Mitel phone at this small office was for free calls. We could just call the extension, but some people may forget to do that. So, I thought the Mitel system could be programmed in a way to stop calls going to this one external number, and route them to the extension. In the way you describe below this would only change some of the number, is it possible to substitue an entire external number for an extension?
Thanks again.