we have bought 3 phones to test with skype for business (VVX 310, VVX 410 and VVX 500).
With all phones we have the same problem.
When the telefon rings and I pickup the handset, the ringing doesn't stop and I'm in the status to start a new call.
I can only answer a call by pressing the Line key when the handset is on the phone.
Serie: 3111-43162-001 Rev.:A
welcome to the Polycom Community.
The phones would have not been shipped with UC Software 5.5.0 and as mentioned in the Skype for Business FAQ and the Release Notes UC Software 5.5.0 is not a supported software for Skype / LYNC.
Please downgrade the phones, factory reset them and sign into Skype / LYNC again. Not knowing if these are brand new or used we cannot confirm if they have pre-configured configuration preventing the phone from answering a call via lifting the handset.
There is a post in the VoIP FAQ documenting this feature.
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services