Zeb's correct, you will usually be required to either continue with the contract(s) until they expire naturally then give notice as per your agreement(s), or pay an amount that is equal to the outstanding balance left to run on each contract term..... in order to terminate the contract(s) early.

Have a look at your contracts to see the expiry dates to work out the figures, then you will be in a better position to understand which is the best route for you.

As far as I am aware you would be obliged to fund the cost of the contract until the end of your current contract. If the 1 year or 2 year contract you originally signed up to has been completed then you are not under any obligation to stay with them....unless you have taken upgrades and new phones which the providers usually offer once a year or 2 years (depending on your contract) to keep their clients retained.

If you have any questions about what penalties may be imposed then you will always be best to approach your provider and ask. Often they may offer an incentive to keep your business but most likely this will tie you into another term contract