Specifically paragraph 5 5.4
That would imply that you have to pay for what you used, as any offers or discounts would not apply.
If free calls were offered, this would be open to abuse.
What a load of rubbish you would never get This from O2 .... Worst mistake made changing to Bt but only took me a week to realise
Yes you do, from the O2 website.
2. The Change-Your-Mind Period – If you change your mind about certain Equipment (usually a Mobile Phone) you've bought from us, you may be entitled to return items once and exchange them for another within the 14 day Change-Your-Mind Period. D etails will be in your welcome letter or email, or in paragraphs 10, 12 and 13 of the Agreement. You can also ask in an O2 shop or check our Website. If you've taken Equipment from us under a separate Device Plan and you are withdrawing from that Device Plan after the Change-Your-Mind Period, you will have to pay in full for your Equipment. You agree that Services will begin immediately if you already have a SIM Card or when you receive a SIM Card. You'll have to pay for any calls, texts, data and other charges you've incurred, including during the Change-Your-Mind Period. .
This applies to all mobile providers, otherwise people could just keep swapping providers with 14 days, and get free calls and texts.