By default, your phone cannot make calls to premium rate numbers. However, you can easily enable the 'Premium rate calling' facility on your Network settings page.
The cost of calling premium rate numbers consists of two parts - 1pMobile's access charge (20p a minute) and a service charge set by the organisation you are calling. For more information, please visit our Tariff and charges page. To find out the minimum and maximum charges of calling a particular number, you should visit this Ofcom page.