Did / do you actually have any phone signal? Is the phone offering you a choice of networks (AKA Carriers) to use?
The phone system in the US is often a bit different from what we have in the UK, and it may be as simple as your phone is not compatible with the specific network you are trying to use (eg doesn't support the needed frequencies). You might want to try selecting a different network.
You may find it's simpler to go buy a 'burner phone' and use that.
