do you have a copy of the docs and under Centralized Voicemail there is a good section there on this?
I personally have not done this but from reading these docs you may be missing the cos option 265.
From the docs:
Extinguishing the Message Lamp
Retrieve the voice mail messages from the extensions used above. The voice mail device will send the digits FF2XXXX to the PBX to extinguish the lamp on the called extension. FF is the Send Message FAC, the digit 2 indicates that the lamp is to be turned off, and XXXX is the called extension number.
Retrieving Messages
Retrieve the voice mail message by dialing the Call Sender of Oldest Message code or by pressing the message waiting key on the set.
When the access code is used, the request is directed to the voice mail hunt group. The system looks at COS option 265 for which speed dial index to go to for voice mail.
When the message waiting key is used to retrieve a message, the COS options listed below are required so that the request is routed correctly.
261 - ONS Voice mail Port
265 - Voice mail System Speed Dial Index
These options must be enabled in the class of service of the Message Waiting Port of the Host PBX and in the E&M trunk class of service of the Remote PBX.
Retrieving Messages - Host PBX
When the message key is pressed, the PBX checks the COS of the device which sent the message. For a message left for a user in the Host PBX this device will be the message waiting port. The COS options contain abbreviated dialing index entered in COS Option 265 so the message retrieval request is directed to the voice mail hunt group via modified digits and abbreviated dial digits.
Retrieving Messages - Remote PBX
When the message key is pressed, the PBX checks the COS of the device (trunk) which sent the message. For a message left for a user in the Remote PBX this device will be the E&M tie trunk. The COS of the E&M tie trunk contains abbreviated dialing index entered in COS Option 265 so the message retrieval request is directed to the voice mail hunt group via modified digits and abbreviated dial digits.