The technology used depends largely on the age of the building:

Many DIY-oriented residents use external hardware to "smarten" old systems:

The hardware inside the unit—ranging from a simple button panel to a corded landline or a smartphone app—where residents receive the signal.

These use a phone line to call a resident’s designated phone number. The resident presses a specific key (typically 9 or 6 ) to trigger the door release.

An electronically controlled latch that disengages when the resident "buzzes" the visitor in. Types of Buzzer Systems