The early implementations of MPT1327 in America, used ACSSB (Amplitude Compandered Single-Sideband), but later installations are FM as used in the rest of the world, and your list of radios rules out an ACSSB system.
As for the shrieks, definitely haven't heard anything like that. It is possible that it is something like ANI (Automatic Number Identification).
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Just getting into the 'wild guess' department.
Do any of the radios have a display like the attached image?
Or a label on the case similar to the following lines
Code:
xxxx
260 / 2401 / 266
7600
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Comint