@KSA88 : It's not the SIM currently being used that matters, but the phone it's in. Is it an EE phone? If so, it'll need to be unlocked from EE before it can be used with a US SIM.
Where did you yourself get this device itself originally? Did you buy it yourself new direct from EE or from a 3rd-party reseller or is it 2nd-hand? The unlocking process & costs are different. Was it originally on contract or PAYG? How old is it? EE won't unlock contract devices less than 6 months old.
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To phone EE CS: Dial Freephone +44 800 079 8586 - Option 1 for Mobile Phone & Mobile Broadband or Option 2 for Home Broadband & Home Phone
ISPs: 1999: Freeserve 48K Dial-Up > 2005: Wanadoo 1 Meg BB > 2007: Orange 2 Meg BB > 2008: Orange 8 Meg LLU > 2010: Orange 16 Meg LLU > 2011: Orange 20 Meg WBC > 2014: EE 20 Meg WBC > 2020: EE 40 Meg FTTC > 2022:EE 80 Meg FTTC SoGEA > 2025 EE 150 Meg FTTP
It's not as simple as that I'm afraid, Unlock a UK / Euro iPhone and Bobs your Uncle for the use in the rest of the World.
Post GSM and 3G, a lot of Regions went there own way for 4 and 5G Radio Bands, in particular the USA which use some of their Bands are unique the the US and Canada, whilst you may get some form of Service it may be very basic or non existent outside of the City Areas.
My advice would be to try and find which US Carrier you intend to use and then double check with the iPhone Model number as not even the US Networks are comparable with each other in the same State.
