Thats what i've been trying to find out,i've sent O2 two emails through their website and got no response, so I called Tesco Mobile and got through to some woman that didn't know the difference between her ancles and her elbows, she said 'I'm not sure really err' They might be',so I said can't you do your job properly and find out and the call just cut off, and she sounded like she was stuffing her face! Some people are soooooo lazy
However, the same situation applies to T-Mobile and Virgin as Virgin Mobile is 50% owned by T-Mobile UK and the other is half is Virgin Media. Both charge off-net prices when calling between the two brands. Yet when I had the T-Mobile mates rates 5 day pass, I was able to use the free call allowance to call Mobile World users.
And another thing, the two companies are ripping is off really because T-Mobile does not charge termination rates to Virgin, so they have no excuse to charge 35p per minute and T-Mobile has no right to charge 40p per minute.
Same applies to Hutchison 3G(3-Mobile) when you go out of 3G coverage, it automatically choose the O2 or Orange 2G network as a backup, but the companies still charge the off-net rates.