eggplant

I have a customer using Phone Manager and analog phones.
The problem i am having is that some users often get the following.
"You can not access voicemail due to multiple logins"
I can see the user is connected to voicemail in call status but they are not connected on the phone or in phone manager.
It can take several hours for the voicemail to drop the call so they can't access voicemail.

Any suggestions would be appreciated.

Reseting the voicemail service is not an option as the customer uses VM Pro heavily.