payg or contract

Hi, I have an o2 payg sim and I'm on the unlimited tariff. By topping up £15 a month I get unlimited calls and texts to other o2 numbers (and obv you can use the £15credit), so if like me most people you know are on o2 it works out very cheap. Also if your buying your new phone from the o2 shop, if you take your old phone complete with charger you get money off your new phone or credit on your new phone. Plus, if you've been on o2 for year i think it is, you get money off for 'upgrading'. I got £30 off my new phone!
Hope you get sorted
Scarlett x

Thanks for your advice, I really appreciate it!