Call forwarding unconditional

Try using the follow-me feature in freepbx instead.

That way you can configure each extension to go to a mobile after first trying the internal extension, and set a destination (such as voicemail) on time-out.

kspare,
problem is the way the FreePBX dialplan treats CF calls. In it's current implementation the phone will ring for the ringtime specified for the extension or the system default otherwise. After that, it will end the call and go to voicemail. This is 'controversial' in that some schools of thought say 'if you call forward, it should go there and never come back' while others say ' it should simply use that as the number to ring but continue to honor the other settings (ringtime) and then continue on as always. Neither one is right or wrong, but the current mode is how it has always been so changing it would be a feature request, a setting that made the system act differently either overall or per extension. There are ways around this but can be tricky. You can set a very high ringtime, but have the phone itself redirect the call to voicemail after a shorter time with some phones. This results in CF calls ringging much longer, but if not forwarded, the phone itself can create a much shorter ring before failing over to voicemail. That solution does get fairly 'complicated' to administer though.
You could consider asking TB to implement the above described feature and then submit it back to FreePBX for inclusion in a future release:-) (and of course put it in our feature request bug tracker so the thought does not get lost when we are reviewing features)