Executive Resolution not yet addressed, cannot reach agent

I was contacted by XM Executive Resolutions to resolve a billing issue whereby I was auto-billed several hundred dollars for roaming charges... during a time period in which I was not out of the country... on a mobile line I had ported out of XM multiple days before the charges started appearing!

Suffice to say, those charges need to be reversed and after initially hearing from the executive resolutions team via email, I have tried multiple times to reach this person at his direct line and he doesn't pick up, and his VM inbox is full, and he doesn't respond to my emails.  What do I need to do in order to get some time/attention paid on this issue?!

[Edited: Personal Information]