Things above a T bar ceilings are considered "accessible" but they
need to be supported by structure. Most doorbell transformers go in a
box KO and as long as the box is properly attached it is OK. The other
issue might be the cable itself if this space is used for return air.
Then everything needs to be plenum rated.
That is the most frequent violation other than loose cables laying on
the tiles. They need to be supported too. They do make clips for that.
(attach to the ceiling support wires).