Voicemail counter is wrong if xtn not registered

In order to give people a personal voicemail box and a department one, I have created a sip xtn (8001 in this case) and added this to peoples monitored voicemails, it works fine… but…

the output of “sip show peer 8001” shows the voicemail count as “LastMsgsSent : 32767/65535” which is nonsense. So I logged a softphone into that account and as soon as it had registered, the counter returned to normal (0/0)

So, I dont want to have a couple of phones configured just to keep these counters correct, so is there a way to make asterisk count the voicemails even if the xtn is not registered?

(reason for this is the use of a script to display mailbox counters on a wallboard)