Can I trade in old T-mobile phone to get ETF cover?

I used to be T-mobile customer, and I switched to Sprint, and going back to T-mobile.

I'll be switching 3 lines, can I trade in my old T-mobile phones?(two flip phones, one smart phone)

if not, can I just go buy cheapest prepaid phones and trade in? I'm not looking for payback, I just want ETF cover.

I asked rep on chat, but answer was very vague.