Im on O2...... I think they, like most others, really get their arms in for calls which arent prepaid. When my contract concludes, I intend to switch to '3' and make use of their calls being to any network and anytime
I switched from Orange to O2 about 2 years ago and have never looked back.
Whilst Orange undoubtedly excelled in the area of Customer Service (IMO), their unreliability (signal would sometimes vanish for 2-3 hours in areas where there was good coverage), lack of coverage in my area, and high call rates put me off.