We've been using T-Mobile for over 15 years and LOVE THE SERVICE!!! Speaking of service, they are always very helpful, polite, and often point out a better plan that may have been introduced since we last talked with them. They have the best customer service I've encountered post-covid. The only con is that we don't have robust coverage in the sparsely populated areas along Interstate 90 in the West. We expect that to resolve in the near future.
Set your phone to wifi-calling to make free calls or pay 25cents/minute using cell. We went to our account online and set our international coverage to the setting that allows international use, but blocks transactions that "are not covered in our plan" (Cellular at Sea). That way, we keep our phones on while at sea to catch every fleeting moment of coverage, but avoid being charged for Cellular at Sea. We've sailed the Norwegian Fjords without cell interruption (except when out-to-sea), off the coast of Spain and Gibraltar on a TA, South America, the Caribbean and throughout French Polynesia. You will be absolutely amazed how far offshore you get a decent signal.
Text and data are free. Use free wifi (when safe) to save cell minutes, if you will be burning data. I almost never switch to free wifi on land, since I don't stream. My DH does switch to conserve cell minutes, since he occasionally streams. We were getting a fast 5G (instead of 3G or 4G) signal throughout our October cruise from Amsterdam to Bordeaux to Barcelona. Amazing!
We never purchase wifi on a cruise because we do everything we can while near or on land. If you must connect while at sea, then you need to find a solution for that segment.