how much do you charge to call a client in private chat.

[camshow]

I have been asked to call clients while in private. I have refused because I couldn't figure out how to get the phone apps to work or how to block my number. Long story short i figured out how to block my number and thought calling those that request it couldn't hurt. How much should I make them tip for me to call them or should it just be an extra since they have me in private?