Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If you switched phones on that line, I presume you found that the other phone worked fine and received calls there.
If the line is OK but a phone on that line is not receiving calls, and other phones on the same line do fine at receiving calls, then the phone that doesn't receive calls is defective. It cannot be a fault of the phone system box at the other end of the line - other phone stations do fine on that line. It cannot be the fault of the line, for the same reason. Nothing is left but the phone station itself to be at fault.
And there is not any 'mode' to set the phone in so that it ignores incoming calls.
(Looking that up to be quite sure ... I find no overt mention of such a function.)
The only possible things to check before either discarding the phone, replacing it, or having it serviced (esp. if it is young enough to be within warranty coverage), would be to make sure you are plugging the phone line into the right jack on the problem phone. In the wrong jack, it might ignore incoming calls on that line.
The right jack, if there are two, would be the 'Line' or 'L' jack.
Let me know what you think... I may have understood some points about the case not quite correctly, I understand. Thanks.