Cancellation due to conditions of contract change without saying

Hi!

I write here because I have the feeling that at least people here is much nicer that in the shop.. Where they were not helpful at all. And not nice either! So the thing is, after 2 years paying contract and phone, we were thinking that then, we only had to pay the contract. But out of nothing, the contract went up in price (without any notice) and now we are paying the same BUT with the difference that we are only paying service, not phone. We asked for this and the answer was "yes, that happens, so if you want to pay less for the contract, then get a new phone".. really logical, when my phone has not given me any problem! So, we were checking and we found vodafone has a contract for couples, so we both can move and pay way less than now. So my question comes here: I will cancel the phone, mine and from my husband, but we want to conserve the number. We are inside the period still of 3 months in advance. So, I know we have to send a letter, this I know, but:

What should I say in the letter?
Should I add the documents from the contract information?

Do you need any extra documents? This is important because we are not leaving the country at the moment.

The thing is, we have an all flat net M tarif. Do you know if there is any other that we have not to pay 40 euros each for only 1GB and call? There is any tariff I miss that O2 can offer so we save money here? Because this is the main issue for the change!

Hope to know soon from here!
Thanks

Sandra