Ip intercom solutions allow access to intercom calls through multiple viewing platforms.  Allowing 2 way communication providing an addition layer of security allowing all visitors to be screened before being allowed initial access to a building. 

Telephone entry systems allow visitors to communicate with residents and be granted access for residential buildings.  Low cost straightforward intercom solution for residential buildings. 

Already have wires and a dated intercom system? In some cases it can be more cost effective to repurpose this infrastructure.