Oldest Compatible iPhone

I’m signing up for a multi line shared data plan and when I went to activate some old iPhones only one worked, a 1st gen iPhone SE. I had some older iPhone 4/5 that don’t have sim trays on the side and there wasn’t an imei number in settings like there was with the 1st gen iPhone SE. The 4/5 only showed a meid number. 

Anyhow, can anyone tell me the oldest iPhone that is still compatible with Xfinity mobiles shared data plan? I would greatly appreciate the help. I tried calling and online chatting with customer service but they couldn’t tell me the answer to this and directed me to a physical Xfinity store but I don’t have time to drive nearly an hour each way to ask a question that I’d imagine would only take a minute to answer.