On the AV-OS units, only analog phone lines can be used. Digital phone lines can damage or destroy the AccuVote's internal modem. It is unknown at this time as to when we would be able to produce a modem internal to the unit that could handle digital phone lines. There are adapters available to connect an analog modem to a digital phone line. I've used them before. The problem is that they require more to set up than just switching it on and that's more than we want the pollworker to do. There might also be "digital line capable" modems available to plug into the AccuVote's serial port. You'd have to redirect your communication from the modem to the main serial port. I don't know of the existence of a "digital line capable" modem but I'm sure that could be looked into if attaching an external modem to the AccuVote was an acceptable solution.
On the AV-TS R6 units, the modem's capabilities are limited only by what's available in PCMCIA card format, and the price you're willing to pay for it. There may be ballot station software considerations to account for, but others could answer that better than I.