I have a unlocked iPhone 4 or at least that's what I've been told when i bought it. But then I went ahead to unlock it again, when I put a SIM card in, it says no service or at least i hope that's what it's suppose to say. Any hoo I just updated my phone to ios 7.0.3 or whatever and I tried putting in someone else's Verizon and Sprint SIM cards in but it will not get past activation. It says SIM is not valid but when I put the SIM in for activation, but it says no service, which is a good thing. I've been told to try T-Mobile or ATT&T SIM card and reboot it to get past activation. I have just purchased a Straight Talk SIM card and I am hoping to transfer over to them and hoping the SIM card works or any SIM card to get past activation. Any advice or a solution to my promblem?
Phones are a bit tricky. On one hand, the manufacturer (Apple) should make it clear how to use the device, but since you get a no-service error that part of the equation seems working well enough.
If the device shows no service with several known good SIM, you might need to check with Apple or the carrier that originally locked the device to verify it's unlocked + test with an active SIM from that carrier.
If you can use the phone on wifi, it's activated (in the technical sense - activation is a specific step needed before the phone software starts to operate) and now you can focus on establishing cellular service and troubleshooting that portion of iOS: