If you have a delivery door that's locked, chances are you miss
deliveries since you can't hear, or aren't there to hear, the
knocking of the delivery person. Intercom systems enable you to
overcome that problem.
GSM stands for Global System for Mobile Communications and is
the world's most popular standard for mobile telephony. Here in
the U.S. it is used by AT&T and T-Mobile as well as the services
that contract to use the networks of these two companies. Other
providers in the U.S. such as Verizon and Sprint use a
technology called CDMA (Code Division Multiple Access).
Given that GSM is more widely used across the world, and cell
phone network intercoms have been popular in other countries
such as Europe, at this point only GSM intercoms are available.
Manufacturers want to develop equipment that can be used by the
widest variety of markets so they can quickly recover
development cost and make a larger profit. These intercoms have
not been available in the U.S., but are now becoming popular
here as well, especially since analog landlines are starting to
disappear as confirmed by these statistics: