IPhone Activation

I have an unlocked IPhone that was used in the UK and I'm trying to use a local SIM in the US. When I insert the SIM it says the Activation is Required. It then says the SIM is incompatible. I can see the local SIM carrier on the Settings of the phone, and it shows cellular and LTE service at the top left. THe US carrier has confirmed that the phone is comptabile with the service (both GSM). I have searched and don't see anything online about needing to deactivate a phone and then reactivate it according to region -- only to unlock it from a network. Has anyone had any experience with this? My next step is to call up the UK carrier again, but I want to be clear what I'm asking for as I can't find any information. Are Iphones locked to countries, as well as to networks?