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.