Hi, thanks for your question. My name's XXXXX XXXXX I'm going to assist you with it.
When a consumer enters into a mobile contract, there are essentially two contractual elements (one for the handset and one for the service). If the contract is entered into online, the Distance Selling Regulations (DSR) will apply.
Under the DSR, a cooling off period begins when the contract is made. It is possible for the cancellation periods for the goods element and the services element to differ. In relation to the handset, the cancellation period begins seven days from the date the goods are received. With regard to the services, the relevant cancellation period will depend on when the services are agreed to commence and when the consumer is provided with a set of written terms (which are required by law). Such terms must be provided in a durable form and contain details of the services, costs, payment, right to cancel, etc.
If this information is provided within three months of the day after the contract was concluded, cancellation rights will last for seven working days after the information is received (unless the parties agree that the services will start before the end of this period).
If this information is not provided, the right to cancel ends three months and seven working days from the day after the contract was concluded.
Therefore, if the services have not yet started or, the services have started but these terms have not yet been provided, cancellation under the DSR is still permissible. The items will need to be returned by recorded delivery along with a letter that rights are being asserted under the DSR within the cancellation period provided by law.
Please find some further information below from the OFT (particularly from para 3.30: http://www.oft.gov.uk/shared_oft/business_leaflets/general/oft698.pdf
I hope that's helpful but please do let me know if I can clarify anything.