MMSI are formed in such a way that the identity or part thereof can be used by telephone and telex subscribers connected to the general telecommunications network to call any of these objects automatically.
As of 2024, there are six kinds of maritime mobile service identities: An MID consists of three digits, always starting with a number from 2 to 7 (assigned regionally).
A listing of MIDs assigned to each country can be found in Table 1 of ITU Radio Regulations Appendix 43.
In the United States (one of whose MIDs is 366), federal MMSIs are assigned by the National Telecommunications and Information Administration and are normally (but not always) formed as 3669xxxxx.
If 50,000 boaters decided to fit small Inmarsat M terminals, 50 additional MIDs would be required to accommodate them.
This is especially egregious for hand-held VHF devices, for which a single MMSI number makes little sense.
[opinion] The World Radio Conference, Geneva, 1997 (WRC-97), adopted Resolution 344 concerning the exhaustion of the maritime mobile service identity resource.