Person A calls person B. Person A states that the "call may be monitored" and person B, then states "I do not consent to the recording of this conversation." Can person B still record on his/her own behalf, and use this recording in a court, and not have to worry about person A using the recording in a court.
Stating that you do not consent does not have any real effect. Under California’s all-party consent law, consent is required only if there is a reasonable expectation of privacy. See Kearney v. Salomon Smith Barney, 39 Cal.4th 95:
“if a business informs a client or customer at the outset of a telephone call that the call is being recorded, the recording would not violate the applicable California statute”
“if a Georgia business discloses at the outset of a call made to or received from a California customer that the call is being recorded, the parties to the call will not have a reasonable expectation that the call is not being recorded and the recording would not violate section 632”
this provision does not absolutely preclude a party to a telephone conversation from recording the conversation, but rather simply prohibits such a party from secretly or surreptitiously recording the conversation, that is, from recording the conversation without first informing all parties to the conversation that the conversation is being recorded. If, after being so advised, another party does not wish to participate in the conversation, he or she simply may decline to continue the communication.
Having been informed that the call may be monitored, the fact that you continue to speak during the call implies consent on your part. That is a two-edged sword; the calling party likewise does not have a reasonable expectation of privacy, so by informing you of the possibility that the call is being recorded, they have implicitly consented to you doing likewise.