The usual rules apply about cancelling a contract for a service. You first need to check if there is a binding contract. If you have paid a deposit, this would indicate there is a binding contract. If there is no contract to buy a service, you can change your mind about having the service and will generally not have to pay anything. You should contact the mobile phone company immediately, saying you do not want to go ahead with the service and confirm this in writing. You should keep a copy of the letter. If there is a contract, you cannot usually cancel the contract and decide not to buy the service simply because you no longer want it, cannot take advantage of it or you do not like it. However, there are situations where you may be able to cancel a contract:- if the mobile phone company with whom the contract has been made has not broken any term in the contract, you could cancel it but will have to compensate the company for any losses it incurs. The contract may include specific terms about cancellation which may include a set charge. For example, some mobile phone companies operate a 'cooling off' period which would entitle you to cancel a contract without costs even if there is no express term in the contract, you have a statutory right to cancel it if you bought services costing more than £35 away from the mobile phone company's premises or if you signed a credit agreement anywhere other than on the trade premises of the creditor or supplier if the mobile phone company has broken her/his side of the contract, you may be able to 'rescind' (stop) the contract.
For further information, call in or telephone your local CAB.
Please note that this answer is correct as of November 2011