bored@work: For specific details you need to ask your Rov. I am not a Rov so I can only raise questions, not give answers.
But, I would not lend my own phone out, even for a good cause where I could not verify that no high cost services nor international calling was being accessed.
It is apparent that when I was growing up that my teachers were significantly stricter in issues having to do with potential Gezel (theft). Also, everyone was much more careful with their own money. Hopefully the recent economic events and the Madoff scandal will be a wake up call for us all.