I have a monthly tarif with o2 that includes unlimited texts & data. Can I use this with an unlocked iphone? ?

I have a data plan witb unlimited texts and the free web bolt on. I wanted to use thi with an iphone as I want to buy one but the amount of texts per month is ridiculously small. I want to know the risks of tryin gthis. Wil it work? I know I will have no visual voicemail but I don't mind that. What are the pros and cons? Can I buy an old one and upgrade the software?