Even though the unit was designed for standard analog lines (PSTN or POTs lines), the unit also works well with many VOIP lines. In our lab, we tested Comcast and MagicJack, and the unit worked well with both.
Some clients have reported having difficulties programming this device using a specific VOIP or PSTN line. However, once the unit was programmed using another phone carrier, the unit was able to block the numbers that were programmed when connected to the same phone lines.  So, if you have any issues programming your blocker using a particular carrier (PSTN or VOIP), we recommend that you try using a different carrier to program your blocker. Once programmed, connect it to the original phone line and it should work fine. We highly recommend MagicJack VOIP line to program your unit as we tested it and had no issues.