Tipping is always up to you. Your agreed donation covers your time with the lady but if you want to show your appreciation and tip is always appreciated and you will be rewarded in return.
Our ladies are just people like you and appreciate any extra thought you do. And its not all about money. I have seen girls come out with a big smile on their face with chocolates or flowers or a bottle of wine in their hands. That little extra be it gift or monetary will make them remember you and tell the phone girl to put some stars beside your name for next time. Moves you to the top of the list.