I've had a similar issue.
Our users were getting a "number unavailable" when ringing a specific number, but when ringing the same number from their mobile phones, it rang fine.
Based on that information, I assumed I had an ARS problem - this was a cluster of MCDs so there were internal IP trunks involved for least-cost routing). Checked ARS, everything looked fine. Put in new routes so calls to that number were not sent over the IP trunks but went straight out to the carrier locally. Same result.
So then I googled for similar numbers that were identical right down to the last two digits. One was a private home, one was a business.
The first dialled fine & I got ring tone. The second rang fine and was answered by some kind of ACD.
So I decided there probably wasn't anything wrong with the phone system after all and ARS was working fine.
I ran an EDT trace on the trunks. Made a call to the suspect number and saw a "congestion" message.
So the conclusion was that calls from one carrier were getting a congestion message from the remote end, while calls from a second carrier were not getting that issue.
Eventually managed to talk to lvl3 support at the carrier to get him to run a trace and he saw what was happening: the remote end was answering the call with a D channel renegotiation message. It just so happened this carrier didn't support that.