Under Michigan law, it is legal to record a conversation as long as the person recording has the consent of one of the parties to the conversation. The consent, or knowledge, of all participants is not needed. If the person recording is part of the conversation, then he has his own consent, which is all that is needed. If a caller hears a warning that the call is being recorded, and continues with the call anyway, that could be viewed as giving consent to the recording.
The remedies for a violation of these rules are limited. If you are charged with a crime the violation might prevent the call from being used against you at trial. You might also be able to get an injunction to prevent future recording. You might be able to sue for money damages, but it is unlikely to be worth the cost, since your damages will probably be small.
I also point out that your belief falls a bit short of actually knowing or actually having proof. If all you have is your belief, it is likely that any lawsuit you file might be quickly dismissed.
Contact me at 248-399-6930 for a free consultation. You and I do not have an attorney-client relationship formed by our communications on this website. Advice given by me on this website is general advice based on partial information. You should not rely on any advice given without first hiring a lawyer in the area where the case is pending, and providing that lawyer with full information.