Wierd Problem

Got a weird one here. Logically it doesn’t make sense.

Extension 382 is a remote extension connected to asterisk box P(olice) via a DSL and VPN.
A call from 382 to any other extension on box P is OK. Good Quality etc.
Box “P” is linked to box C(ity Hall) via a sip trunk.
Any call from 382 to an extension on box “C” is OK.
Box “P” is connected to the outside world via a PRI.
When 382 calls the outside world outgoing audio breaks up.
Calls from extensions local to box “P” to the outside world are OK.

The first thought is a bandwidth problem with the DSL, but the problem does not affect internal calls, only PRI calls.

By the way, this configuration worked flawlessly until late last week. No changes have been made in the configuration.

I know there are folks out there that are smarter (and more sane) than me. Any ideas would be appreciated.