The whole point of calling a phone-sex line is that you know what you’re getting, right? You pick up the phone, pay your money and partake in some, um, self-satisfaction. It’s a sure thing. Everyone hangs up happy.

With the comedy "For a Good Time, Call …," you only think you know what you’re getting. It looks like total formula and — for a little while, at least — feels like it, with its broad types who are complete opposites getting thrown together in a contrived, high-concept situation: operating a phone-sex business out of a Manhattan apartment they’re forced to share.

‘FOR A GOOD TIME, CALL …’ Rated: R Opens today at Consolidated Ward

But the actresses playing the two lead characters — Ari Graynor and Lauren Anne Miller — have such a light and lovely chemistry, and director Jamie Travis keeps things moving so briskly, you find yourself not minding how by-the-numbers the story is. And then within that by-the-numbers story, there ends up being enough surprises and subversive twists that you find yourself unexpectedly charmed.

Miller co-wrote "For a Good Time, Call …" with her college roommate, Katie Anne Naylon, who really did run a phone-sex line out of her dorm room because she needed the cash.

Comparisons to last year’s hugely successful "Bridesmaids" are inevitable, given that it features women saying and doing the sort of raunchy things that previously had been the staples of Judd Apa­tow’s bro-centric oeuvre. They actually finished their script before "Bridesmaids" had even been shot, but the two films do share a wonderfully honest exploration of close female friendship, and how discovering another woman who truly "gets" you can carry all the thrills of falling in love.

Graynor and Miller’s characters are miles apart emotionally, though, at the film’s start. Graynor’s Katie is a brassy party girl with a wardrobe full of animal prints and a stripper pole in the middle of her living room who pays the bills through random jobs. Miller’s Lauren is conservative and precise, all headbands and innocent nighties, a young woman of privilege who had her entire life mapped out — including, she thought, marriage to her longtime boyfriend, Charlie (James Wolk).

Then Charlie coldly dumps her, calls her "boring" and kicks her out of their apartment before moving to Italy for the summer, leaving Lauren with no place to live. When her best gay pal (Justin Long, doing it up big but not too big) suggests he has a roommate for her, she’s shocked to find it’s Katie, her college nemesis. But Katie is just as desperately in need of a roommate to avoid being kicked out of her own place.

And so voilà. We have a premise.

Naturally, they clash at first, but eventually learn to tolerate each other. Then, when Lauren discovers that Katie is moaning and groaning into the telephone for one of her many gigs, she realizes that with her business acumen, they could both make a whole lot more money. And naturally, Lauren will eventually end up answering the phone herself in a rebellious shedding of her good-girl image.

Yes, there are the obligatory montages as the women take in the calls and rake in the cash, but the graphic one-liners and the lightning-quick editing keep the laughs coming steadily.