Block up to 100 outgoing phone numbers, area codes, and prefixes | Can be programmed to block "UNKNOWN", "OUT OF AREA", and all other calls with no caller id | Blocks up to 1,200 unwanted phone calls, and 10 area codes. Works with land lines, VoIP, DSL services, and digital voice services | Simple, one-touch programming to block annoying phone calls - FOREVER! | No batteries required! Units run on the power from the phone line.