Jack works 9 different jobs, most part-time, and switches between the 3 identities. Now, Tyler = Jack, right? With that in mind, as soon as Jack picks up the phone- Tyler and Marla immediatly stop f*****g. But how? If Jack (Tyler) was downstairs answering the phone, what was Marla doing upstairs screaming like that if she wasn't actually having s*x with Jack (Tyler)? And if you apply that logic to al of the s*x that takes place throughout the film- it all works.