Paul McCartney and John Lennon worked together closely for years. When The Beatles broke up, though, their friendship deteriorated. Lennon and McCartney fought often over business affairs and they could hardly have a conversation without shouting at each other. Lennon began to screen McCartney’s calls, which the latter found very hurtful.
John Lennon would screen Paul McCartney’s calls.
Throughout the 1970s, McCartney reached out to Lennon in an effort to rebuild their relationship. This was made difficult by the fact that Lennon rarely believed it was actually his bandmate on the other end of the phone. He made McCartney answer security screening questions before he agreed to talk to him.
Source: imdb.com