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Note the Presence button. For me, the Presence list is reflected on my Polycom phone. I think this is a good place to help manage the phone's screen.

The earlier advice from Anirban is certainly correct, but for me the Presence screen is a better fix.

I have found one reference which helps to configure username of polycom vvx phones displayed in all lines.

https://support.ringcentral.com/s/article/Name-Displayed-on-All-Lines-of-Polycom-VVX-Phone?language=en_US

Check if this helps to fix your issue.