If it said £15 for 500 mins, it could have been simply giving an example of what the current labs bag can get you:
£5 goodybag + £10 credit = £10 / 2p/min = 500 mins
Just a thought? 🙂
I think its advertising the existing labs bag - first look was just a banner 500mins for £15 - looked again later and its a rolling screen with 2p a minute/1p a text explaining the 500 mins for £15.
Next look was a banner for a months free dental treatment - must be all that talking ruins your teeth!
PS dental treatment was a different company. 😉