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.
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.
The only known problem currently is with some MagicJack units in which you are unable to input your passcode when calling into the unit. This is a problem caused by MagicJack. For more information, read the article Arming/disarming system over phone doesn't work with MagicJack or other VoIP service.