This is to do with the fact that the 9xx series software has its own onboard dialplan. This dictates (controls) what digit streams the phone can and cannot send.
You have two choices. You can change the dial plan or, since several phone types can't send a trailing * (Mitel, Siemens and several others) and since this is also true of the 9xx dialplan, SAIL will accept an 8 instead of a trailing *. Thus you can dial *508 to access voicemail. This works for all SAIL feature codes.
Kind Regards
S