Normally these causes would give a busy signal, but off the top of my head.

1. Tandom CO is not enabled in COS
2. When the call is forwarded from an outside number, the Toshiba system push the caller ID of the incoming call as the outgoing caller ID. This can cause some carriers to not allow the call because it is not a DID on their lines.

Is it a PRI or a regular T1?