RE-SEND: FollowMe - Ability to insert a "+" when extending off-system

Previously marked as being added to v15 on: FREEPBX-14469

When I asked in that ticket if it had actually made it into v15, I was told to open a new feature request instead, so here it is...

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It would be helpful if we could hit a checkbox that would allow the inserting of a + on the originator's Caller ID sent outbound when a Follow Me call is extended to a user's outside number (such as a cell phone).

This would allow some sort of indication to the user that the call is being extended from their deskphone, while still maintaining the originating caller id.

Basically, when someone calls my desk, and Follow Me is active, my cell sees their Caller ID like this: "513-777-3456". My hope is that with this checkbox, the display would now show "+513-777-3456". This would be a helpful indicator that the caller is being extended from my desk phone.