I've seen a lot of discussion about cell phones and wifi on the forum so I thought it would be useful to report our very good experience with T Mobile, which is our carrier.
The international plan allows free texting and wifi -- but throttled speeds, and charges 25 cents a minute to make and receive calls.
We enable wifi calling on our phones, and had good experiences with calls in Oban at our accommodation. We also use What's App to stay in touch with family at home, with friends along the way, as well as travel vendors in Scotland. That works very well, too.
We were on western most tip of Skye -- a couple of miles from the road at an iron age defense. I wanted more info about it, and got it -- via wifi on the spot.
We were on A836 between Ullapool and Strothy -- seemingly in the middle of nowhere -- and received a What's App call, staying connected as we were driving for about ten minutes. Not long after that, we got another call via TMobile and stay connected for the duration of the call.
Full disclosure: I haven't been a big fan of TMobile since it acquired Sprint, our old carrier, because coverage at home is not good. But here in Scotland, it works!