Most VoIP services that you could plug a home phone into will be compatible—such as MagicJack, Basictalk, Vonage, Ooma, etc. Simply connect a phone line from the VoIP device to the OUT LINE jack on the system.
The only known incompatibility is that the system cannot receive incoming phone calls when connected to a MagicJack.
Phone service from cable and internet providers such as Comcast, Time Warner Cable, Cox Communications, Charter Communications, AT&T, and Verizon are also compatible. Simply connect a line from the cable modem box to the OUT LINE jack on the system. If you have the cable box connected to your home phone wiring, simply connect a line from a wall jack to the OUT LINE jack on the system.