An Access Control System provides a method for a User to present some type of personal identification (Entry Code, Card, RFID tag for example) which is then compared to a programmed database to verify whether that User is to be granted access at the entry point on that day at that time. If the proffered credential matches the information in the database, the system will activate a device (such as a Relay) which will then unlock a door or activate a gate operator to grant the User access to that entry point.
Some access control systems have a telephone feature that allows guests to call tenants. The tenant can then grant access to facility by pressing a DTMF (Dual Tone Multi-Frequency) key on their phone. If denying access, the tenant simply hangs up the call.