Hi guys, thanks for the responses so far.
There are no mailboxes on either controller for numbers 421 or 221. However each controller has a 0 mailbox. These did both turn out to be full (as no one had any idea they existed!). By making room on these boxes the caller no longer receives the 'mail box full' message however prior to that step the call does not appear to go through to the specified console operator either (when the hold music is playing). This is the principle issue as we don't really want callers to be dumped at a mailbox, we want them to reach a console operator.
The supervised transfer setting is set to false which would be backed up by the fact that when a regular desk extension is specified (rather than a console number) is transfers correctly without further intervention.
So, any ideas on why the consoles don't receive the calls when regular desk extensions do?
Thanks again guys