By default, your phone cannot make calls to high rate premium rate numbers. However, you can easily enable the 'Premium rate calling' facility on your Network settings page.
Premium rate numbers are deemed to be calls costing in excess of £2.50 per minute, or texts where a chargeable reply is received. They usually start with 09 for calls and a short code of five digits for a text.
The cost of calling premium rate numbers consists of two parts - 1pMobile's access charge (5p a minute) and a service charge set by the organisation you are calling. The company you are calling must state the charge on all of their literature and on their website.