Whats done is done, better to tell him the truth.
On the other hand, why are you using BT to place calls to Pakistan when there are so many other cheaper options out there? Try using calling cards or one of those pinless direct dialing companies. We use Raza.com and we get 2 hours for $10.... really cheap and no balance wipeout like calling cards.
Oh they offer this discount number 7p per minute....I'll buy calling cards from now on. that's the best option i guess..or may be skype.