What Happens to an old cell phone after the number is transferred to a new cell phone?

I'm purchasing a new cell phone and I'm planning on transferring my old number to the new phone. However, a friend of mine is interested in buying the old phone from me and before I do any of this I'd like to know what will happen to the old phone's number if I transfer it to my new phone. Will he have to transfer his number to that phone, or will it keep the number it already has regardless of me transferring it to the new phone?What Happens to an old cell phone after the number is transferred to a new cell phone?
When the number is transferred to your new phone, your old phone is deactivated. He can transfer his number to it or he can choose to get a new number.What Happens to an old cell phone after the number is transferred to a new cell phone?
If it's a GSM phone (most carriers other than Verizon or Sprint), the number is in the sim card.