I have a VoIP crisis on my hands. Our field guys dial in using a modem to upload data at the end of the day. It will not work on VoIP. It's standard Hayes stuff dialing a VPN. I'm anxiously waiting for a test bed and crossing my fingers that not a lot more of our thousands of people will order VoIP in the meantime.
To put in into modem terms, the VOiP gear supercedes flow control no matter what the modems want to do, so I don't think that's gonna' work for you. Also you might look into the issue of modem vs. VOiP latency. The "acceptable" latency on VOiP is 150 ms. which may be unacceptable to your modems. I've not looked into this specific kind of problem, so these are just suggestions.
It seems if your field folks have the bandwidth for VOiP they should also be using IP to reach the VPN, rather than dialup modems. To test it, buy a cheap VPN appliance for your end, and reconfigure the VPN software on their end to use IP. I think you'll be chucking all those modems after you give it a run.