How do I know for sure if my phone has been hacked

I received an email in my junk email stating my phone had been hacked, I called this number 833-864-5**0

that was on the email. The man told me I needed to go somewhere and buy 2 ITune cards worth $50.00 a piece, he wanted me to keep him on the phone while I done this so I could give him the code on the back . I hung up on him and did not do what he was asking, they tried calling me back but I didn’t answer. What do I need to do ? I have already changed my AppleID password ....