You would like to have the Message Waiting Indicator (MWI) for a shared voicemail account be activated on multiple phones. You can accomplish this by creating the shared account as an additional line on the desired extensions.
Create a User called "Shared Voicemail"
Create a separate device for each extension that wishes to monitor this account. Assuming that the shared voicemail User is 100 and this needs to be shared by extensions 200, 201, and 202, then you will need to create three devices for User 100 (eg. 100, 100a and 100b).
Add one of the User 100 devices as an extra line on the main device for each of the other Users (eg. User 200 will have device 100 as an extra line, User 201 will have device 100a as an extra line, etc.).