A couple of things come to mind:

You need to change the COS of the lines used by the VM ports. COS 7 or 8 IIRC will block any outdialing on trunks.

There are several area codes that you can block to prevent calls to them. Sometimes your dial tone provider will block them for you. They are searchable on the web, or try here

Finally, I have sent you a PSNA bulletin of the command line commands for the TVA system. OPTN 10 something will let you block a user from dialing 9 as an extension. The clever trick in an unpatched system is to dial 91n as an extension and when the transfer is completed, the hacker can dial 9 more digits to complete the call. It won't show up on SMDR but your really long holding times on an incoming call will be the key....until you dial tone provider sends you a bill.

Carl