Cheapest cell plan I've ever had was 5GB LTE per month, unlimited talk/text for $20 a month from Mint Mobile (have to buy a year at a time for that price).

I dropped them after a year because they didn't support iOS WiFi calling or visual voicemail.

iOS is finally a fully supported phone so i'm switching back.

Backend network is T-Mobile.

https://www.mintmobile.com/ios-features/