I was looking around for a particular use case where
* employee visits abroad and carries a cell/mobile phone.
* employee wants an extension forwarded to that phone while he is out of office.
* Admin can allocate an extension and set the destination number as his call/mobile.
* Admin can also set an expiry date on the extension.
Once the extension is created, calling in to the system or calling internally to that extension will forward the call using specified international routes (that are used when calling from an internal phone). When you dial the extension past the expiry date, "The extension you dialed, has expired!" message is played back to the caller (this is to stop people abusing the system).
In this case the trunk cannot be set for this particular extension alone. All calls will go through the trunks/outbound routes that are defined for calling from an internal phone. If this module proves useful for folks around, I can spend some more time and improve it as per suggestions/feedback.