Stupid question... Have you verified the COR of the caller can place the call? For example, if the caller is coming in on a PRI trunk, is the COR of that trunk allowed to make a call out the outbound trunks?
Here is how I would trouble shoot this...
1) From an internal phone, dial the speed dial number. Is the call successful? Then the Speed Dial is OK, if not, your speed dial is setup incorrectly.
2) From an internal phone, dial the ACD Path and allow the call to Interflow to the Speed Dial, if the call works then go to the next step, if not, check the interflow settings and adjust the ACD Path parameters until it works.
2a) If the caller is not dialing a DID directly to the ACD path, but an Auto Attendant or being transferred by an operator, call that extension from an internal phone and duplicate it.
3) If all this is good but the call still fails from outside, change the speed dial to internal number and test it. If it works, then the most likely issue a COR problem.