You can (as you probably know) run the phone using HFA firmware.
To test a phone for MWI
DIS-WABE:GEN,,,,MBON;
DIS-WABE:GEN,,,,MBOFF;
Using an analog phone only - dial the MBON code and then the extension number to turn on an MWI light
Using an analog phone only - dial the MBOFF code and then the extension number to turn off an MWI light
Then you will know if the phone will indicate or not for your correct MWI On/Off sequence coming from Digital or Analog Voicemail etc.
To avoid confusion also you should turn off the Callback light - CHA-SBCSU:STNO=XXXX,CBKNAMB=NO;
(Callback no answer stored in the mail box)