You can use any carrier that has the coverage you need but the key to having low monthly cell bills is to NOT sign a contract.
You will need to use your existing equipment or purchase new/used equip on the open market and even flash/debrand it if it is not already done and you wish to do it. I don’t know if this can be done with i-phones. There are quite a few services that become available on a feature phone if it is flashed (unlocked) which are unavailable under contract or the carriers CHARGE you monthly for (such as wifi, for example).
I AM aware that a 2-yr basic i-phone contract from AT&T (where you buy the phone from them) ends up costing the user $1800-$1920 over the life of the contract ($75-$80 mo), PLUS the initial cost of the phone. I’m not sure if there is any way to get basic i-phone svc cheaper if you were to “acquire” an i-phone (new or used). Any Piggs know?