Add-on 
 | 
Data 
 | 
Minutes 
 | 
Texts 
 | 
Price 
 | 
All in One 15 
 | 
All-you-can-eat 
 | 
300 
 | 
3000 
 | 
£15 
 | 
All in One 25 
 | 
All-you-can-eat 
 | 
500 
 | 
3000 
 | 
£25 
 | 
Given that the standard price for calls is 3p/min, what's the benefit of the "All in One 25" add-on if the "All in One 15" (£15) + 200 minutes of calls (200*3p=£6) would cost £21?
I tried asking Three, but they just replied, "Hi Rob, you've not missed anything, that's the correct price of our Pay As You Go Add-ons."
Update:
So, it seems that Three's response to my question was to cancel the "All in One 15" add-on and replace it with an "All in One 20" add-on, which has the exact same minutes, text and data, but costs £5 more. #facepalm
