Praise be to Allah.
It is permissible for the one who has credit on his phone to sell it to someone else, for the same as it is worth, or for less or more, because this comes under the heading of selling benefits or usage, and it is not selling money for money, such that it is stipulated that the amounts be equal.
So the seller or owner of the credit owns a usage or benefit of making phone calls, the value of which is 10 pounds, and it is permissible for him to sell it for 12 pounds, and there is nothing wrong with that. The same also applies to selling phone cards; it is permissible for one who owns a card worth 100 to sell it for more or less than that.
And Allah knows best.