I've had this trouble before and I can't figure out what the fix was.

I have a repartee 7.45 VBPC DNIC for Mitel. As far as I can tell, I've done everything but the m/w light will not come on. I can see the ports go off hook and dial the *41150Q where *4 is the feature access code, 1 is for m/w on and 150 is the extension and I think Q is hang up. but no lights.
It also dials to turn off the lamp after I check the message.

In the phone system setup form, *41,X,Q is the string for m/w on and *42,X,Q is the string for m/w off. I've dialed each port individually and they all answer. Ports 4-8 are set to A/D. 1-3 are just Ans.

I've made a test phone with the same Mitel Class of Service as the voicemail and I can light/extinguish the lamps just fine. For some reason the vm dials the string but the lamp does not light.

Phone systems for NJ and NY businesses