Hosted Exchange UM Fails from an internal source

Hi All,

We have a Lync 2013 Server setup on prem with Exchange Online Hosted UM.

Everything is working as it should apart from users dialing voicemail (holding 1 on their IP phones, or using the Lync client to change greeting etc) - these all fail with the following error from Lync reporting:

22; reason="Call failed to establish due to a media connectivity failure when both endpoints are internal";

I am having real difficulty tracing the culprit.

Everything else works, external access to SA, users can leave voicemail and it emails and light up the client/phone appropriately.

Any thoughts?

Thanks

Lee