Sorry if we already know this, but just read this on the Carphone Warehouse site:
So, like other phones work with transfering numbers (in the UK), you could have to wait 5 days till the number switches across. A bit annoying when you can't even put your old sim in the iPhone for those few days.
I think in America it took a matter of hours to port a number, not days!
New & Existing customers
If you're an O2 customer billed by Carphone Warehouse, or you have a mobile phone with another network, you'll need to contact them to obtain a "PAC code" in order to transfer your number to your iPhone.
This process may take up to five working days to complete.
Please be aware that PAC codes are only valid for 30 days.
If you're an existing O2 customer and you receive your bills directly from O2, or you're an O2 Pay & Go customer, during the activation process you'll be asked whether you'd like to transfer your existing mobile number to your iPhone.
This process can take up to 5 working days to complete, but don't worry - we'll provide you with a temporary number so you can start enjoying your iPhone straight away.
So, like other phones work with transfering numbers (in the UK), you could have to wait 5 days till the number switches across. A bit annoying when you can't even put your old sim in the iPhone for those few days.
I think in America it took a matter of hours to port a number, not days!