Personally, I would set the CM trace to Detailed and run a test call. Then I would pull that trace down and look specifically for Media resource allocation errors (if your codec mismatch theory is accurate) and the digit analysis (to ensure that the CSS you think should be used is actually used).
Do calls fail no matter what directory number is called? (i.e. what happens for MVA-to-IPPhone vs. MVA-to-2ndStageDial)
I'd also check the CM service parameters under the section (System-Mobility) to ensure things line up.
Sorry, not much help I know. But I would start troubleshooting by checking configs and pulling the CM trace.
HTH.
Regards,
Bill