The two parameters that control message waiting stutter tone and visual message waiting indicator are MWIEnable and X_VMWIEnable, respectively.
The "Message Waiting" parameter is not actually a setting. Confusingly, it is just a dynamic yes/no state indicator of whether or not a message is currently waiting. Don't mess with it.
There are also two parameters (again, hopelessly confusing) in ITSP Profile A--SIP (or whatever profile you are using) that may need to be set. I use Callcentric, which doesn't use these parameters, so a voip.ms
user will have to answer whether or not voip.ms
now requires them. These parameters enable "subscribing" to a voicemail server which controls the message waiting status indicators.
See the attached screenshots.