No Service after 5 seconds

I actually ordered a Samsung phone from Amazon, but got sent an unlocked iphone 6 instead. I got a sim card for it, and sprint prepaid says the transfer took.

The problem is that after I restart the phone, or after 'calling' ##873283#, it says Sprint at the top with 1 bar, stays like that for ~5 seconds then goes to 'No Service'. Never been able to make a call with it.

The Wi-Fi works fine, but phone has no data plan so can't test it on data outside of WIFI.

Is there anything I can do to troubleshoot this?

Thank you for your time.