Phone Charges

My company had all employees sign an agreement that the employee would pay any overage charges on their cell phone resulting from personal calls. This was signed back in 2004. The company has never charged for the over charges. Then last month they decided to give a bill for the overage charges from the last four months, October thru January, without any prior notice that they were now going to start enforcing the agreement. Can they do this?? I was told by a friend that there is some law that if they don't enforce an agreement or rule for a certain amonut of time they can't start to enforce it, out of the blue, with out any notice. Is this true?? The bill they gave me was a lot but probably the same monthly average amount over since I got the phone. Do I have to pay these?? How far back are they allowed to bill us, if at all?? Could they essentially tell us to pay the back bills since 2004 now??

Thanks

If there is that much a of a time lag you can argue that it simply is not fair and you should have been warned. You could very possibly win in small claims on that point so the employer may compromise with you.