Hi, just try by adding user name onto server address setting.
(Polycom field help message)
Enter a line identification address that the phone uses to register with the server. The address may include a user name, the host of the phone’s SIP URI, or the H.323 ID/extension. For example, if the phone’s line is firstname.lastname@example.org, enter 1002 as the SIP where polycom.com is the server. Or, you can enter email@example.com. Any address entered will be displayed as the phone’s line if the display name and label are not specified.