My phone is so old I don’t have a plan.
I dread the day they stop supporting it or the battery dies.
Batteries are available with replacement tool kits for all iPhones on Amazon and eBay. Depending on your model, it can take as short a time as two minutes to replace the battery. Videos on how to do it are on YouTube.
The real problem is the carriers are planning to phase out 3G someday. 2G is almost gone now.