Add a field to set up a mobile phone for each extension that will sit in the DB. Preferably, create a checkbox to allow or not allow the user to change this setting from inside his voicemail settings (like the possibility for the user to do follow-me today from within Recordings).
With that number on file, we can:
a) Change automatic sending to VM to be replaced (user toggleable!) with some sort of "Press 1 to leave a message, press 2 to be diverted to the callee mobile phone, press 0 to reach an operator".
b) use the number (again, user toggleable, and since money is involved, also admin forceable to NO), use the SMS[1] function of asterisk with the PSTN SMSC with that mobile number as a parameter to notify a user for a new stored voicemail (similiar to the text being sent today for new messages by e-mail, just to the cellular phone). If we are REALLY going further here, a speech-to-text would be neat, but that's probably for freepbx 4.0 or so ;-)
[1] http://www.voip-info.org/wiki-Asterisk+cmd+Sms
