Does anyone know how/if i can keep my mobile number but change to a new sim card on orange?

i have been given a new orange sim card that i want to use as it gives me free texts however i don't want to change my phone number. i'm already on orange and have been told there are ways of keeping your number with a diferent sim..any helps welcome! (u.k only! well if your'e not from the uk you can help if you can but i was just thinking orange is a uk based company and i'l shut up now!!!)Does anyone know how/if i can keep my mobile number but change to a new sim card on orange?
i would talk to your mobile provider and see what they recommend...