I love the slang term "meh", even if I try my best to not actually use it in conversations. Sometimes, you just need a word for "not good, but not bad either", all in one. Kinda like Goldilocks' "just right" porriage; just right unflavored oatmeal seems to be "meh" to me. T-Mobile's international roaming was one such 'meh'.
I have Sprint, transitioning to T-Mobile. I had to make a couple of calls over to Scotland to clean up my hostel reservations. That cost me significantly. Sprint/T-Mobile also has Global Roaming, where one can get free texts and free 2G-equivalent data. You can make this work for texts and Emails, but trying to pull up an interactive website or Maps wasn't fun.
All modern phones have nano-SIM chip cards in them with the information needed to connect you to your cellular network. Since the iPhone XR and XS, iPhones (and most new Androids) support eSIMs, which are virtual SIM cards stored in the phone itself. This lets these newer phones have 2 cellular connections, for example one US carrier and one UK carrier. A good document for iPhones is here on Apple's site. If your phone is unlocked (or unlocked for foreign cell providers), you can have a primary and a secondary service.
When I bought my iPhone 13 mini (more next post about that), Sprint/TMobile put their service on an eSIM. Since I bought it from Apple, it's unlocked. Before the next trip out, I will go buy a physical SIM card or an eSIM one-month plan from some network provider, and activate it before I need to make any phone calls for the trip. When I get over the ocean, I'll turn the UK plan to primary for data and phone, so I can make phone calls if needed and can have 5G data. However, the US number can stay active for texts and emergency phone calls as needed. When I get back, I delete the eSIM and discontinue the plan.