So your customer is not using a 0/8/9 for an outbound prefix? That isn't really a supported setup and can potentially cause all kinds of issues, like you see now... You are probably just seeing the beginnings of problems, they will likely multiply. :/
How to avoid it?
Change the COR of the trunks so they can't dial out.
Change your Trunk Service to absorb some digits, adjust DID routing table, and make it an invalid outbound call. For example, if your DID is 343-1234, absorb 3 and have it route in the DID table off of 1234, if 1234 does not exist it will route to the invalid number intercept since "1234" will (should) not be a valid ARS match.
Add all DID numbers to the DID routing table, regardless if they are in use or not, and set ones that are not being used to fixed point (speed call to extension, RAD, voicemail, AA, or whatever).
Probably more options, that's just off the top of my head.