He says that he has number forwarding enabled from his condo to the headquarters.
Thats how even the insurance agent can reach out to him

Because he gave her his new phone number during one of his 'black out'.
Tyler Durden is having is own life from a long time. Maybe before the beguining of the gilm. That's why the hero thinks he can't sleep and is so tired.

Oh good, that's clears it up. Thanks.

Its a service you could set up through your phone company back then, before cell phones and internet. So when someone called your old number the call was automatically forwarded to your new number.
I'm a Gunslinger. I deal in hard calibers.

Sorry about how ignorant this sounds, but how does a forwarding number work?

the phone conversation goes as follows -
Marla: "where have you been the last 8 weeks
Jack: "Marla? how did you find me?"
Marla: "you left that forwarding number"
I'm a Gunslinger. I deal in hard calibers.

None that I can think of, he might've called her at some point while he was Tyler and she saved his number.

jackdurden.com makes perfect sense to me and it explains unclear things in the movie.
Maybe Fincher just wanted the audience to figure it out themselves?
You have to admit the idea the movie is more than meets the eye gives it a little more depth.

That's fair, but I just wish the movie established it, you know?

I'm not interested in how accurate or plausible the jackdurden.com theory is, because it's just a fan theory. I don't mean that as an attack or to take it down a peg. It's just that I don't really care for it, and it's not confirmed. There's a lot of fan theories I've heard that I like, some about Fight Club itself, but I don't actually believe any of them to be 100% fact.

"^^ delusional moron"
well, you're obviously a moron yourself and in denial as well. Do you have something to back your bullsh!t up?
please explain to me how exactly is jackdurden.com wrong?
and no sarcastic stupid jokes please, just logical explanation.

^^ delusional moron
Norton's character probably updated his phone number where he went for his group sessions.

No it's the only explanation. Isn't good enough for you? This just happens to be exactly what the movie is about.

Is there any explanation outside of the jackdurden.com theory?

because Marla is imaginary. So is Paper street.