firstly I would not recommend to use UCS 3.3.1 as this has been updated and the current supported Release is UCS 3.3.5.
Can you always reproduce the issue?
Does this phone have a sidecar and/or BLF?
How does the issue manifest itself (Show the call on the display but unable to answer)?
In order to officially troubleshoot this you would need to raise a ticket with your Polycom reseller to bring this to the attention of Polycom support.
You will need to provide the following:
Wireshark Trace of the Phone booting up until issue is reproduced (if reproducible)
Phone Configuration files
-app.log and -boot.log as a complete set (via Syslog or FTP server with append allowed)
SIP Server Details
Notice: This community forum is not an official Poly support resource, thus responses from Poly employees, partners, and customers alike are best-effort in attempts to share learned knowledge. If you need immediate and/or official assistance please open a service ticket through your proper support channels. Please also ensure you always check the VoIP , Video Endpoint , Skype for Business , PSTN or RPM FAQ's