RE: MMSI numbers
Good questions, Ken.
Pushing the red button, with an MMSI installed and the GPS hooked to the radio will only send out an undesignated emergency alarm on VHF channel 70. Your radio will transmit the digital alarm briefly on 70 and then revert to 16. The CG will respond on 16 and, just like an all voice event, will ask the same questions. So essentially pushing the red button is a digital means of getting their attention and they will still need to talk to you.
I have an Uniden 525 VHF radio and I can either (a) use the red button to send an undesignated alarm with my MMSI and my lat/long or I can (b) select the DSC menu, select emergency and then select a designated emergency, like fire, sinking, injury, piracy etc. All DSC radios have the ability to send different types of messages, ship to ship, Pan Pan, all ships, position request etc. These other options are accessed through the DSC menu function, not the red button.
You are right about boats changing hands and not having current info programmed, and yes that could be problem. If I were to sell my boat I would darn well make sure I have erased my personal MMSI from the VHF before handing over the keys. I dont want to have someone else sending off my info to the CG. If I bought a new boat one of the first things I would do is to change the MMSI to a new one with my info. To do this some of the radios may need to be sent to the factory for reprogramming.
The only info that is going to pop up on your radio as a result of someone else sending an emergency DSC call is their MMSI number and their lat/long. You will not see their name, phone number or even a description of their boat. This last info is in a database and linked to the MMSI but it does not display on our VHF radios.
It is also my understanding that commercial traffic is only required to monitor 13 (bridge to bridge) or the Vessel Traffic (VTS) frequencies. In Puget Sound VHF 14 is south of Bush Point on Whidbey and VHF 5A is north of Bush Point and includes all of the Strait of Juan de Fuca and the San Juans. This is why DSC is so important. When you push that little red button it sends an alarm to ALL DSC EQUIPPED VESSELS on Channel 70 and then automatically switches their VHF to channel 16 and now everyone is listening to the emergency. If the commercial guys are only listening to 13 or 5A and you are out their broadcasting your mayday on 16 without using the DSC feature they will probably not hear you at all.
All fixed base VHF radios manufactured for sale in the US since 1997 have been required to be DSC capable. Very few handheld VHF radios have the DSC feature.