What is the resouning behind placing CME at the branch location instead of the PSTN location?
Are there any users at the PSTN site?
In this scenario if the WAN connection is down, you have no backup to make outound calls. If the CME was at the PSTN location at least users at that location would be able to make outbound calls.
Ideally you would have your CME at the PSTN site so you can limit bandwidth requirement on the WAN link, unless like I said there was something preventing it, such as already existing trunks connected to old PBX, etc.
Chris