Chosen Solution

I bought a used iPhone 5s and it was meant to be unlocked. I put in my 3 network SIM card and it says ‘sim not supported’ and it does not let me use the phone. When I put in my O2 network SIM card it lets me use the phone, starts searching however it says ’no service’ in the top left of the screen. What I tried:

  1. I tested both SIM cards in a different phone and they worked.
  2. Resetting network settings.
  3. Rebooting the phone. I did a lot a research on forums but can’t find an answer. The phone gets very good WIFI signal so I don’t think it’s an antenna. And when I put in that 3 network sim it says ‘sim not supported’ however it shows three circles in the top left corner so I’m not sure if that means 3 bars of signal. The IMEI is: 352054062049680

Phones network locked.

Agreed with Ben your phone is carrier locked to 02 or a virtual network that piggy backs on O2. You’ll get No Service in the top corner when the O2 sim is in the phone until you perform a carrier settings update. Go to settings and about then you should get a message pop up to install one (when connected to wifi) or connect to iTunes a click check for update. If you are still not getting any service the you need to look at other issues.