There are several types of commercial intercom systems available to cover most applications.
There are basic units that usually include a remote station with a “push-to-talk” button but there are also more complex units that work through cell phone service and still others that provide customized signaling to indicate distress.
In a basic unit, the primary function is simply voice communications.
These devices are generally located at access points to permit visitors and other personnel who are normally restricted from entry to obtain permission to enter an area.
In the more complex specialized units, the telephone system is connected to signaling devices such as emergency phones on a long bridge or in a tunnel.
A variation on this style is college campus systems that provide a talk path but also signal a security station that someone at that location is in distress.
Many of these systems are also fitted with signal lights to make the emergency location easier to find for response personnel.
Some of these systems also have alternate power sources. They contain batteries and solar panels that can be automatically recharged by sunlight, eliminating the need to run power cables to each commercial intercom station.
When such units are coupled with cell phone service, the designer can place emergency telephones nearly anywhere they are needed