iPhone 4s - can't send texts but can call and connect to wireless?

recently purchased the new iphone 4s on O2 i had to transfer my sim to a micro sim and since then the sim hasnt worked properly. I only have a few contacts so far but none of them work to send texts apart from one which is the exact same phone as mine and entered in the iphone field. However there is an older version of the iphone entered also but that doesnt work even though its in the same house as mine -.-

i cant seem to figure this out i live in the UK so the area code stuff is either two things and i have tried both on all of the contacts that dont work and nothing has worked i've even tried removing the area code altogether or setting them as 'other' instead of 'mobile'

the carrier settings is set as automatic and if i turn that off nothing happens :S

have tried taking the sim out and putting it back in

have tried turning the phone off and on

have tried resetting the network

at the start i thought it was O2 because i only had 1 bar which confused me because i could call them with that but it just wont send a text :S HELP