Re: MMSI into Icom IC-M504
There are two different issues here.
The first is the MMSI number differences between a Boat U.S.-issued one and one issued by the FCC. The first type is not entered into the ITU international database and thus is not available to, e.g., foreign search-and-rescue entities.
I know of no specific prohibition to a boat with a Boat U.S. - issued MMSI communicating by voice with a foreign vessel.
HOWEVER, there is a prohibition of communicating with foreign vessels or in other countries without having two things:
1. a commercial radiotelephone Operator's Permit; and
3. a Station Lcense for the boat.
Both of these (the Permit and the License) are issued by the FCC upon application. Along with their issuance, you automatically are assigned a MMSI number, and this number should be used in your VHF, HF, AIS, etc.
So, strictly speaking, if you're off Cove Point in the Chesapeake and want to talk with a foreign-flagged LPG carrier via VHF you need to have the FCC-issued operators permit and station license.
At least that's the way I understand things.