That’s the whole point of buying your phone unlocked: to allow competition between carriers. Growing up in Europe we had very cheap prepaid plans but you still paid more for out of network than in-network, so lots of budget conscious people had two or more prepaid sims and swapped between them. No monthly bill and you add money to whichever number is low. It helped that receiving calls or sms was free, so you could ring someone and have them call you back if you were a bit low on funds.

Man thinking back, I probably got away with less than 2 dollars a month back in 2006-2010 era.