If I understand what you're asking for correctly (effectively to use a modem over a voip service?), I think you're going to struggle to find someone able to do it, as most voip systems don't have the quality for it - e.g. see
http://www.soft-switch.org/foip.html - it's about faxing over a voip network, but it's effectively the same thing.
I guess you might be able to get some way of sending the actual data rather than raw modem signals, and so effectively actually having a modem at the other end. It would be easy enough to do, you're just tunnelling a serial port but I'd be surprised if there was a market to make doing it worthwhile...