How to find an iPhones carrier?

My dad got an iPhone from a friend that got it from their friend because they upgraded. The whole phone has been reset and was given without the sim.
I now can't get onto the phone because I need a sim for the right network and have no way of knowing what the network is.
Can anyone help before I end up buying 20 sim cards just to try them all out.

AFAIK, there's no way to know.

You can try taking it to an Apple store and see if they can help you, but I'm not sure about that.

You fail to provide the version or model number of the phone. The model number will tell you at least if it's GSM or CDMA. Then take it from there. Here's a hint because you are so clueless: Verizon and Sprint are CDMA. Other carriers are GSM.

Use common sense and ask dad's friend.

Don't you get it-you are blocked!

IPhone customer service

Let's start with what you mean by "can't get into the phone"? Even without a sim card, an iPhone can be started. It just won't allow any calling. It can be used like an iPod, or if in a Wi-Fi area can connect and use anything that doesn't require a phone number. If it won't do that, it has been blocked because it was lost/stolen somewhere along the way. If blocked, it is a door stop only unless you are given the password by the real owner.