IPhone 5s stolen, retrieved, and got replaced. What can I do with the stolen/retrieved one?

IPhone 5s stolen, retrieved, and got replaced. What can I do with the stolen/retrieved one?

Added (1). It was stolen
I reported it stolen to T-Mobile
They sent me a new one for $175 under my insurance
I retrieved the old one because I found the thief
Can I give the old one to my mom under the same company plan? Or will T-Mobile think we scammed them?

No, since you reported it stolen it will be a bad esn so you won't be able to use it anymore. You can use it like an ipod touch though on Wi-Fi.

Asurion is well aware that people try this as a scam to have a second phone to sell. THE ONLY thing you can do, to make the "stolen" phone usable is to contact the insurance company, tell them the "story" and here is what will happen, They will send you a prepaid box to RETURN the REPLACEMENT phone to them.

Once they get it back, they will release the OLD phone from the blacklist so it can then be activated. Should the REPLACEMENT phone never arrive on their end, you will be charged the FULL retail price for it, and you will be left with the "stolen but found" phone, still with the bad ESN/IMEI that can't be activated. Oh and ONLY the original account hold of the "stolen phone" can get it un-blacklisted.