"Steven P. McNicoll" wrote:

Although many use RCO, Remote Communications Outlet, to refer to any remote
FAA radio, it's actually a tool of FSS. Terminal ATC facilities use RTRs,
Remote Transmitter/Receivers, and Centers use RCAGs, Remote Communications
Air/Ground facilities.

And a newer service, the GCO, Ground Communication Outlet, maybe used to contact
either FSS or Clearance Delivery. Click your radio transmitter slowly 4 times
to contact Approach Control (Clearance Delivery) or click slowly 6 times to
contact Flight Service. The GCO will connect to the appropriate facility via a
telephone line, and when they answer you may transmit/receive to them via this
phone patch. GCO's are not intended to be used from the air.

They all appear to use the same frequency, 121.75 Mhz nationwide in the USA.
GCOs allow a direct line on the ground in places where it has not been possible
to place a RCO due to their need for an expensive dedicated 24 hour/7 day
landline. The approach control option may not be available at all GCO
locations. After a period of time without receiving a transmission the GCO
will timeout and disconnect the phone patch.

GCOs are listed in U.S. Government flight information publications in their
terminal charts (instrument approach plates) and in the Airport/Facility
Directory.