This device works with dial tone (DTMF) protocol. And its working depends on your phone company. If it sends phone numbers to call recipient that it is OK. The device simply detects and analyses phone line activity and detects incoming callers phone number which then is stored in EEPROM memory.
Device can remember up to 10 phone numbers in PICs EEPROM memory. Numbers can be erased at any time. Project files are available for download.