A detective keeps calling me, what do I do?

A detective from the town I lived in a few years back keeps calling me, and the first time, I talked to him. He just said "you're not in any trouble, your name just came up and I'd like to ask you a couple questions" but he wouldn't tell me what the issue was or how my name was attached. Today, he called and left an aggressive voicemail saying that if I didn't get back with him, that he'd "move forward" and "a conversation would go a long way to determining whether it's a misdemeanor or a felony". The only problem is, I have no idea what this is about! What do I do, just sit and wait? I'm just very nervous about this and could use any advice given. Thank you for your time.