Is it legal to record someone without their knowledge?

Of course it depends on what you mean by record.  If the person has a reasonable expectation of privacy, such as a locker room or bathroom, you cannot video tape without violating several laws and also be subject to civil litigation. I'm assuming that you mean to make audio recordings of phone conversations.  Iowa is a one party state, meaning that one of the parties in the conversation must know of the recording if the conversation is conducted in Iowa.  Thus a person can call you and record the conversation as long as they knew they were recording.  This issue can be complicated with cell phones as the cell phone is mobile and can actually be present in a state other than where the home state is located. Some states are two party states (requiring both parties' permission and knowledge) and a few states are all-party states (requiring all parties to the conversation to have knowledge and permission.)