Intercom calls to Extension go to VM

I have a customer that has an office with 5 phones and a home office with one phone all the same tenant.

Recently they have a problem where the two phones up stairs can't call the home office by dialing the extension. When they do they get ringing until she picks up the phone or it goes to the voicemail greeting. If she picks up the phone she hears a tone then a busy signal or error tone. The caller hears ringing then the voicemail greeting when she picks up.

Here's the twist:

The phones downstairs all work just fine. The phones upstairs can call using the *99 feature and that works as advertised and they can call the DID assigned to that phone and it works fine. The problem only occurs when dialing the Extension directly. I have tried deleting the phone and recreating to fix corrupt data but It still does it. I created a test softphone and it does it. They are all aastra phones and models don't matter. Some models work while others of the same model do not work.

I am running Asterisk 1.8.17.0
with PBX Manager 6.1.1.12

Thank you for your help