We use Consumer Cellular (
https://www.consumercellular.com) which uses the AT&T network but is much more affordable. We are low-volume users, but I bet they are a better deal than one of the major carriers for many folks. (And they're not just for older folks - they just use people with grey hair in their advertising!). They don't have the international agreements that the major carriers might have, and I can't get a cellular Apple Watch, but for basic cell service we probably pay half what we would pay with a major carrier.
Asa