Howdy folks! I just found out that my youngest son (20) is an INTP! My sweetheart is an INTP as well. It has just occured to me that both of these good men react roughly the same way on the phone: they both lose interest and want to get off the phone after about 15 minutes.
My youngest stated that he quickly runs out of things to say, and there is a definite change in his voice that tells me that he wants to get off the phone (without actually telling me). My boyfriend states that he gets tired and finds that talking on the phone more than 15 minutes drains him. I can also tell it in his voice that he needs to hang up (even though he doesn't state it - because he is so darn polite).
They both have said that when they are face-to-face with someone, than they can talk for hours and hours, but on the phone, they lose interest quickly.