Can i sell my contract mobile phone whilst i am still under contrct with my network?

I have been on a 18 month contract and am half way thorugh it, the phone i have is rubbish..can i keep my contract sim and buy a pay as you go phone and use it in that phone and continue to pay my bills as normal..if i was to sell the phone will the network find out and block the phone from use..i am with orange. many thanks.
Answer
Am on vodafone and i got a samsung d900 on contract. i actually sold it on ebay yesterday so i can put my simcard into a phone of my choice. so in answer to your question, yes unless you are on 3. also a word of warning the phone you are going to put the simcard into has to be either another orange handset or be unlocked.