iphone won't send SMS messages, but will sent iMessage

One of my kids uses an iPhone 3s for texting and game-playing only. I do not pay for service for it.

It can send/receive texts when she is near our wi-fi. There was never a problem, until I gave her my old 4s.

I put all the info from the old 3s onto the 4s.

Now, neither the 3s or the 4s will send text messages, but WILL send iMessages.

iMessage is ON in Settings, and "send as SMS" in ON. Send/receive is set to a gmail address.

What am I missing?