A hotel changed all there analog room phones to a Cetis type phone, they are sure the MWI was working after the set change and now no MWI on analog sets.
There are 5 per bays and no MWI on any of them, the Ringing LED on the Power supply should be flashing when a phones lamp is lit and the LED is not flashing on any of the Power supplies.
One of the power supplies has been replaced and no Ringing LED on it either.
Have you set a message waiting and checked the voltage of the pair, or listened with a butt set in monitor mode? It is pretty easy to tell if there is a message waiting, you will hear it when on-hook or can measure 90-140 vDC on the pair intermittently to light the light.
It is highly unlikely that all the message waiting is "broken"... The most likely cause is that that the phones were ordered for the wrong message waiting type, or the options of the phone were set incorrectly. There are 2 distinct types of message waiting for lights in PBX's... LED (pretty much all AT&T systems) and NEON (pretty much everyone else, including Mitel). There are other message waiting types like FSK, but they are not supported in this system.
What voicemail system? It is also possible that the message waiting port locked up or the FAC got changed, but need more info to go down that road. If internal VM on ICP did you do a reboot?