[ Cell Phones & Plans ] Open Question : Can I be charged more depending on who I text?

I let some guy use my phone to "text his friend". Later someone told me that this guy could have texted some private number and get me charged a whole bunch of money for it. They said it is a cash making scheme and that guy was probably in on it. The number was only one city away so it wasn't even out of state. Is this true? Could I be charged a ton of money for something like that? Does something like that even exist?