This is a standard school science question which, at my school, we studied at age 11. The correct 'scientific' answer is definitely to add the milk.
However, I prefer the suggestion putting the milk in and taking the tea to the phone! (Anyway, there's nothing in the question here to to specify that it's a landline in another part of the house. Perhaps it's a mobile in the tea-drinker's pocket? In which case he/she could pour the milk in with one hand, while answering the phone with the other).
Chris