iPhone calling within a foreign country
How do I make an occasional call, with a USA iPhone, while traveling in England ?
iPhone XS, iOS 12
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If your iPhone is not carrier locked you can buy a local pre-paid SIM card and swap it out into your iPhone XS. If you are on T-Mobile you are probably not carrier locked, but if you are on anything else you probably are carrier locked.
However, if you contact customer service for your mobile carrier they may be willing to unlock it if you qualify, or if you let them know specifically that you are traveling to England. Then you can use a temporary SIM in England for a local pre-paid carrier.
You could also see if your carrier will, instead of using your normal SIM, activate the "eSIM" on your iPhone XS. Then your physical SIM card slot (currently used by your carrier SIM) could be removed and you could then get a temporary SIM in England for a local-pre-paid carrier.
Or you can simply contact your carrier and ask about international roaming so you could call directly from your iPhone, but international roaming charges can get very expensive, and international data roaming charges even more expensive. You'd definitely want to talk to your carrier in detail first.
Here is Apple's guide on the subject:
About cellular data roaming options for your iPhone and iPad - Apple Support