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I've never come accross a set that does not work as a normal VHF untill the MMSI is entered, but then i have only dealt with Icom DSC units.
With (some) units you need special kit to enter the MMSI with others (generally newer i beleive) you do it from the keypad - much better of course.
Useful note: once entered the MMSI can not be changed without manufacturer intervention so get it right.
I don't beleive you can send a distress or urgency call on the DSC unit without GPS connected OR manual position entered. I also know for a fact that if you or someone else has not entered the MMSI number that big red button is as useful as a chocolate tea-pot.
Hope that helps you all!