Is it legal to eavesdrop on another person's phone conversation, when it is on your own telephone?

Sometimes a person suspects his spouse of having an affair.  In an effort to prove it, he secretly attaches a recording device to the telephone.  When he succeeds in recording a conversation with the boyfriend, he then confronts his wife with the evidence or takes it to his lawyer.  The incriminating recording then also becomes proof that he committed the felony of eavesdropping, which his wife may be only too happy to report to the police.

It is a Felony to eavesdrop on a telephone conversation without the consent of one of the parties to the conversation.   New York Penal Law 250.05.

"Eavesdropping" means to use any device to intentionally overhear or record a telephone conversation or electronic communications.

This is referred to as an "offense against the right of privacy," because each person has the right to expect that their phone conversations are private.  It is the communication that is protected, not the telephone instrument.  Therefore, being the "owner" of the telephone does not give you the right to eavesdrop on the people who are using it, even if they are in your own family.

It is legal, however, to record your own telephone conversations, that is, when you are actually one of the people taking part in the conversation.  You can do this without informing the other party that he or she is being recorded.