The answer to your question is both yes and no. Under Georgia law (other states may be different), as well as Federal law, you can legally surreptitiously record conversations on your telephone as long as one of the parties to the conversation is aware of the recording device.
What this means is that you can legally secretly record conversations between you and any other person on your telephone, and you do not have to tell this other person that you are recording the conversation, because you (who is one of the participants in the conversation) has knowledge of the secret recordation.
However, you cannot secretly place a listening and recording device on your home telephone to overhear and record the conversations between your wife (whom you have not told about the listening and recording device) and another person, such as the person she is having the affair with (as this other person also is unaware of the fact that you are listening in and recording the telephone conversation). Such conduct by you violates both Georgia and Federal laws, constitutes a felony and subjects you to being punished with a term of imprisonment and/or payment of a fine should you be found guilty of this eavesdropping.