G711 wont get you all the way there. It may work fine in a lab but even then it wont be reliable. Modems are very susceptible to clock slew, packet loss, & variable delay. I would check with your vendor to see if they have a solution.
I have done this in a lab and have seen it in action in real world scenarios.
I agree with R that packet loss is a big problem. I have spent some time troubleshooting modem calls over commercial VoIP networks (using G711 bypass) and would say that as little as 1 packet lost will cause the modems to retrain. If packets are lost during the retrain procedure the call will be dropped most likely. Variable packet delay can normally be dealt with by using VoIP gateways with a configurable jitter buffer however.