Hey guys I wanna have phone sex with my gf, do u usually go about telling them before hand like on a text, "lets have phone sex 2night" or should I just go about it tonight on the phone, setting the mood etc. and just doin it??

Well here is what I typically do..
ME: Hey what are you up to?
HB: Not Much
ME: Funnnn. ;P
HB: Haha suree. How about you?
ME: Im just laying in my bed. Im pretty damn bored.
HB: Oh etc. etc.
(here is where they usually offer to entertain you, or something of the sort. But if they dont I pull them in with...)
ME: I might have more fun if you come to join me (;

I have used this so many times, and I cant really recall a time it hasn't worked.
Its simple and states your intentions very clearly.

Best way to lead them into it is role playing. Then get more details as you go, be descriptive and details.

For ex: stir the conversation into a pillow fight, then use sexual innuendo such as I'm gonna throw you around the bed, and pin you down, etc.... Then becomes details.... Like I would grab your wrists the pin them down the bed, resting my entire body weight on top you and and try to squeeze myself in between your thighs.... You get the idea. The actual sex part isn't important, it's the details and sensual stuffs that you tell her that will turn her on.

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