The Signal component automatically stats the service application when you call your own account. The same application is started if you dial any *NN number, where NN is any 2-digit number.
The service application checks how it was invoked. If it was launched to serve a *NN call, it uses NN as the function code. Otherwise, it presents the functions menu and asks you to enter the function code.
You can invoke the service application by calling your Account from any device and using the voicemail application Access PIN feature.
You can invoke the service application by using the Auto-Attendant application Service Access feature.