couple points:
getting the actual number of a restricted call is the same at any company; they dont give it out without a court order becasue its not theirs to give out and everyone nowadays wants to cover their ass
second, customer service has nothing to do with and no access to individual voicemails, and anything to do with it other than resetting the password. MAYBE tech support, but i doubt it. AFAIK, this is also the same with any company. (most compaines also have time limits as to how long they will save the individual messages without a repeated “save” command through the VM system)
this does not explain random dissapearances/deletions, im just trying to shed some light