Hi, I'm currently on the O2 unlimited calls + texts to O2 for £15 a month. Family and friends are on O2 mainly so works well for us. The other thing to think about which i have just done is not actually cancelling your phone.
Call the cancellations dept, tell them you want to cancel, they'll chuck lots of deals at you in the end i got mine agreed to free line rental and a new phone if i signed for 18 months. I signed, not paying any line rental and receiving my new phone. I do not use this sim and have e-bay'd the phone, didn't even open the box. Easiest £250 i've ever made. Look at what you can get the most for and then choose that phone. Make sure don't get tempted to keep it, i was but thought not worth the money i could get for selling it. Just an idea