The O2 Tariffs for the iPhone are as follows.

£35
200 minutes
200 SMS text messages
Unlimited data
Visual voicemail
7,500 WiFi hotspots

£45
600 minutes
500 SMS text messages
Unlimited data
Visual voicemail
7,500 WiFi hotspots

£55
1200 minutes
500 SMS text messages
Unlimited data
Visual voicemail
7,500 WiFi hotspots