There is a known Polycom bug that is causing unexpected behavior when a phone's Ring Type is set to Ring Type 14 or above. Specifically, the No Answer Forward Time will not be honored on that phone, causing too many rings or continuous ringing on incoming calls.
The customer can choose any Ring Type up to Ring Type 13 or Ring Type 15 to correct the problem. Polycom has acknowledged this issue and will get it fixed in the next firmware release.
If that does not resolve the issue, SIP ALG is likely the cause of this issue. Check the router list to find out if a router has SIP ALG and if it can be disabled.