Now that the government has (wisely) told cell phone carriers that they must allow customers to keep their numbers when they change companies, the carriers have decided the best way to make the best of a bad situation is to charge their customers fees in case they switch. In other words, as long as you don’t switch carriers, you will be charged a monthly fee to pay for the people who do switch. So the longer you stay with your carrier, the more fees you pay.
Note that this does not hold with all carriers. T-Mobile, for instance, isn’t charging a cent. And with others it varies by state. But you could be paying $0.32 – $2.83 for the right to change your number. Check your bill for exact amounts, and check the link for what other people are paying.