Deregistering a number from iMessage

Hi,

This is probably an odd query, so hopefully there is somebody out there who can answer the question. I bought this on myself, by playing with something I didn't fully understand, so feel a bit stupid right now.

I just upgraded to the iPhone 4S, and had my operator unlock my old iPhone 3GS. When I restored my 3GS, I put in temporarily my work mobile phones SIM card - from another operator to validate that the handset was indeed unlocked, and registered it against my Apple ID. Somewhere along the line I managed to register that telephone number with iMessage, as well as my proper mobile number on my 4S.

Now, whenever I send a message to my contact for my work number - it get's sent as an iMessage, and get's delivered to my iPhone 4S, and my iPad, and to my iPhone 3GS (if I have the other SIM in it) - but if that SIM card is in my work Nokia handset, the SMS doesn't arrive on that handset... only if I turn off iMessage from the iPhone4S that I am sending the message from. I am assuming worst case scenario, and that any iPhone that uses iMessage that tries to SMS message that number will end up using iMessage, and thus I will never receive any SMS messages on that Nokia handset that originate from iMessage enabled phones.

So my question is this - how can I deregister a number with iMessage, and have it so it uses only my apple ID email, and my personal number that is permanently going to be in my iPhone 4S.

Many thanks in advance for any help you can give.