Who must give permission to record a telephone or in-person conversation?
Federal law permits recording telephone calls and in-person conversations with the consent of at least one of the parties. See 18 U.S.C. 2511(2)(d). This is called a "one-party consent" law. Under a one-party consent law, you can record a phone call or conversation so long as you are a party to the conversation. Furthermore, if you are not a party to the conversation, a "one-party consent" law will allow you to record the conversation or phone call so long as your source consents and has full knowledge that the communication will be recorded.
In addition to federal law, thirty-eight states and the District of Columbia have adopted "one-party consent" laws and permit individuals to record phone calls and conversations to which they are a party or when one party to the communication consents. source
But, though it's probably legal, that doesn't mean it isn't underhanded.