take my advice: go for the ad that says $10 for 10 minutes or something. when u get on the phone you can add another 5 or 10 minutes (which 10 is not long enough) for like 5 extra bucks. do it. trust me, not doing it that way can cost you $80 or so.
also, get a clock near you, a digital, just so u know when to be serious and keep an eye on the time.
as far as talking to the girl, keep in mind shes heard it all. the nastiest **** u can think of, its same old same old, she does this for a living and odds are, she likes it otherwise, she wouldnt bother.
so yeah, just tell her whats up. like me, once i was really missing an ex's hot sex. so i tried said fuck it, and called. i told her the truth, i have a girl on my mind, and i wanted to act out a fantasy. will you be her for me?
piece of fucking cake! of course she says yeah, whats her name? what she look like, how does she fuck you? does she suck you good? does she let you fuck her ass? etc. and have her say your girls name, say your name, its actually pretty hot if you hot over someone, u know? tell her you want her pussy, tell her you want her ass, and tell her where u want to cum. in her ass, on her face, whatever.
remember, she doesnt know you, youll never meet her, and shes heard it all. it really can be good.
of course, if youre not looking to fantasize about an ex or whatever, just make sure you're honest and to the point of what u want, dont be shy, youre paying for dirty talk and nothing should be *****! shes's heard it all.