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Cab, keep in mind that they WILL have to eventually move off of the 500 frequencies. If they choose to move up to 700, then they would be mandated to use digital voice (unless I read the mandate wrong. They could also go to 800, where I believe you will still be able to use analog voice. Just some food for thought.
Yes, all but a few 700 "analog eligible" channels are required to be implemented in digital. Digital in the general pool of assignments can be anything that meets spectral efficiency and does not necessarily have to be P25. However, the interoperability channels must use P25.
Thing about 800 is that the top tier manufacturers are only selling digital systems for their new systems and are attempting to sunset the analog systems.
As for conventional vs. trunking, check the Region 28 plan. Sure, they may be able to use conventional, but the region will likely REQUIRE use in trunked configuration, as it's a resource multiplier. I doubt there are "that many" frequencies they are allotted in the plan compared to what they have on Channels 19 or 20 (500/506), so trunking will have to be the way to go.