Hi I have had a problem like yours, where noone could call the PSTN, but only make internal calls. The thing was that the accounting log had completely filled up it's drives space, and thus resulting in the server not being able to account any traffic leaving the local vicinity, and heading to the PSTN. I resolved this just by moving the entire folder to another partition with adequate free space. Not certain if you are having the exact same problem, but I hope it helps!