She spends a few minutes figuring out the functions of it from a software point of view. Lain does type something, but she doesn't send a message to anyone, and does do it in full view of Nagito. It's more that she just wants to see how it types, not that she actually wants to send any messages as him.

Then, abruptly, she spins the device around, and removes the back plate to reveal the battery.

It seems like she wants to take it apart even more, but since it's not her property, she doesn't. She instead nods, puts it back together, and carefully hands the cellphone back to Nagito instead.

"Thank you."