Cannot leave voicemail when calling internally. External to internal calls work fine.

Hi guys,

We've just noticed that internal calls are dropping when not answered instead of going to Voicemail and it seems to be isolated to just 1 of our 7 offices. If we call the same number from a mobile phone the voicemail works fine.

We are running a Hybrid Skype for Business on prem/Exchange Online environment

Handsets are Polycom VVX500's

Each office has its own provisioning server

Thanks in advance for the assistance.