How to text and use iMessage on an old iPhone 4s without having to use Wi-Fi?

IIm getting my phone taken away for a bit and I want to be able to still text or iMessage on an old iPhone 4s. My friend can use his old iPod to iMessage but he has to have Wi-Fi. How can I do it without having to use Wi-Fi or how can I at least set it up on my iCloud?

You can't.