Lync IP Phone issue - cannot connect

Have issues with Lync on a Polycom CX500, using sign in via extension and PIN works fine. The phone logs in and can make calls etc.

Intermittently the phone will go onto the below and keep retrying, occasionally it will sign back in.

After this initial issue the phone will again go onto this screen after a random time period and keeps on doing this, sometime we will pull the ethernet to get it to sign back in.

Any ideas on what the issue may be?

Thanks