Registering iPhone 4 with iTunes but waiting for new number

Hi all,
Ive just bought a new iPhone 4 with a contract with Orange in the UK. Ive previously had an iPhone 3G with O2 and Im waiting for my PAC code so that I can take my old O2 number with me to Orange.
When I plug in my new iPhone 4 and launch iTunes to register it, I need to fill in the phone number. The new phone comes with a phone number already which will be changed in a few days. So should I fill in the number that new phone has (and would I have chance to change it later) or should I register my old number on the new phone (as this is the number that will be used once my PAC code has come through and given to Orange)?
Many thanks in advance.
J
24″ iMac, 2.16 GHz Intel Core 2 Duo, 1 GB RAM Mac OS X (10.6.6)