I think your daughter is old enough to realise that she has to plan her time to include lessons and games and friends. Help her to make a detailed daily time table and see how much of her time is spent in chatting on the net. Explaining only in terms of how the money (now spent on calls) can be spent better is obviously not having the needed impact.
She must give you an undertaking that she will not spend more than 50 - 60 minutes a day on the net, chatting with friends. If she is not amenable to the suggestion, you will have to shift the computer to a central room, where there will not be much privacy for her as some other members of the family will also be in and out. If this is not possible, you will have to say that she is on trust to keep to the time limit. When the next bill comes and reflects her long hours on the chat-line, tell her that she must be prepared to have the modem cut off. Of course, she will sulk a bit, but I cant imagine that it will do a great deal of harm. Children have different levels of sensitivity and you will be the best judge of how strict you can be. Explain to her that her performance in the Boards will practically open or shut doors for all future decisions. She should do as well as she can, without feeling over-pressured. Connect her present activities to her hopes for the future. She will probably tell you that she is discussing her homework with her classmates! Be firm in your rules for her, but have a touch of gentleness and a bit of humour in your interaction. If you can get her father or others in the family also to support your views, it would have a better impact. Her good marks in term exams etc will become a reward for hard work. But she has to feel motivated herself. You can only help it along. 15 is a difficult age anyway. It will be much better in a year from now.