Would it be legal for me to record a phone call, without the other party’s explicit knowledge or consent that it was being recorded beforehand, as long as I, a participant in the conversation, consent to it being recorded, to use as evidence in a federal court case if both calls were made in single party consent states (i.e., from Texas, Connecticut, South Dakota, etc)?
There are two distinct legal questions lurking in this question.
One, in a conversation on the phone between someone in a single party consent state and a two party consent state, is it legal for the single party consent state participant to record the conversation?
Second, assuming for the sake of argument that the recording was made illegally, is it admissible in evidence in a civil lawsuit in federal court?
I'll answer only the second piece.
There is not an automatic exclusionary rule to exclude the admission of evidence illegally obtained by a private individual in a federal civil lawsuit in the way that such evidence would be automatically excluded from evidence in a criminal case if it was obtained illegally by law enforcement.
Instead, the court applies a balancing test on a case by case basis, weighing the circumstances that render the obtaining of the evidence illegal against the probative value of the evidence to establishing a disputed evidentiary fact in the case. The federal judge has fairly wide discretion to admit or exclude illegally obtained evidence in a federal civil lawsuit.