Complete instructions
https://www.ricksteves.com/travel-ti...call-uk-europe
/edited to add reminder/
If you haven't taken the service you now carry overseas, check with your carrier before you go. Different plans range from free data/text/20 cents per minute voice to totally unaffordable. If your plan is more expensive than you want, consider purchasing a SIM (local phone number with a rechargeable data allotment) when you get to your destination. Make sure your phone's unlocked to do that.