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.
Wireless is SO available any more that I can't imagine that you're not moving to that kind of connection anyway. Every hotel I stay in has either wireless, broadband, or both. And you can hook up to wireless outside Kinko's or Starbucks all day long. There's a cost, but you don't have these headaches, and the speed is tremendous, a real productivity kicker. I avoid dial up like the plague unless I absolutely have to use it. Is there something I'm missing here?
Good suggestions, but we have thousands of devices and an InfoSec group that refuses to let anyone on the VPN without an RSA token. Making the device a part of a in-home network also offers challenges. We have serial, USB, BT and 802.11b -- no ethernet. Our next gen device has GPRS which will help eliminate the need for nightly data upload. For the few people that have put themselves on VoIP, we've moved around the problem by allowing them to upload directly via 802.11b when they come back to the facility the next morning.
We'll see how it goes.