I have two iPhone 6plus units. They are both fully functional but have not been used for about 4 years. They were used with ATT. Can these phones be used with other carriers by simply installing a new SIM card for the new carrier?

paulfrombransonwest wrote:

I have two iPhone 6plus units. They are both fully functional but have not been used for about 4 years. They were used with ATT. Can these phones be used with other carriers by simply installing a new SIM card for the new carrier?

No, this needs to be done How to unlock your iPhone for use with a different carrier - Apple Support

In addition to the carrier unlock procedure already mentioned, you shouldn't expect too much by way of battery performance for devices that have been sitting for years. They were most likely completely drained in that duration, and that is the worst way to store a battery.

My experience with various iPhone models is that the 3g, 6, and 6s (both sizes) had the least reliable batteries.

Note that there were at least 2 different models of the iPhone 6, each with different carrier and radio band compatibility.

You'll also be stuck on iOS 12.