I'm trying to find a way to realize a decentralized callcenter, so that the callcenter agents can talk from their home office with just a PC + headset. The big challange is that some of the agents have very low bandwith and the quality with VOIP is too bad. So my question is: What other solutions are possible when VOIP is not a option? What exactly do I need to do if I try to achieve this with for example Landline/PBX or GSM? (e.g. Which software do I need to install? Which device do I need?)