Garmin GPSMAP 580 Owner's Manual page 91

In order to use DSC, you must first register your VHF radio with
the FCC and receive a Maritime Mobile Service Identity (MMSI)
number.
What is a Maritime Mobile Service Identity (MMSI)
Number?
The 9-digit MMSI number is similar to a phone number when used
in a position report, and is a unique identifier to the Coast Guard
when used in an emergency distress call. Garmin does not supply
this number!
How Are MMSI Assignments Obtained?
If you are outside the United States, you can obtain an MMSI
assignment from their telecommunications authority or ship registry,
often by obtaining or amending their ship station license.
How Can Garmin Help You with DSC?
To receive distress calls or position reports, you must have a DSC-
equipped chartplotter as well as a VHF radio with DSC support.
During normal, non-emergency position report communication,
Channel 70 (156.525 MHz) has been set aside as the VHF/DSC
digital call channel.
For a distress call, press the Mayday button on the VHF radio. The
call transmits on an emergency channel with the MMSI number
GPSMAP 580/585 Owner's Manual
attached to the call, if the receiver has DSC support. Any DSC-
equipped mariner (on sea or land) within range can receive the call.
When the GPSMAP 580/585 is connected to a VHF with DSC
support, the GPS unit shows a call list, log, and a directory. Basic
wiring is required for the Garmin unit and VHF radio to send and
receive NMEA data from each other.
Appendix
85