I don't understand the goal. All calls delivered to phone belong to phone and it's user. User is unable to pickup other calls just because they are not ringing on the phone at all.
Yes, you can arrange call pickup - so user can pickup other calls, but it require special configuration. Just don't do it and user will be able to pickup own calls only.
Also, I'm unsure what's role of sidecar you expect here - incoming calls are ringing on particular Line Key of phone, no sidecar is necessary to pickup them. Even Line Keys need not to be used here. Just take handset from cradle and the ringing call become picked up.
