He feels an open summons and lets it grab him -
"It might be easier for you to find me," John puts in, "and get me to find him. Assuming he's not back in Arkham Asylum or still living in this apartment, which are the first two places you should check."
"Okay." Cam picks up the stick-shaped computer he made earlier from where he set it down and makes a note of this - "What's the address of this place, and how do I find you?"
He gives the address of the building. "Apartment 203, but I wouldn't recommend trying the front door if you don't know who's home. The window is safer. You can find me in nice, normal ways, like the phone book and Google."
"Awww, Google. Phone books! Trouble is, I can make stuff, but 'data and phone service on a preexisting network' is not 'stuff', so in case it's inconvenient to acquire same - you don't still have pay phones now, I think, and if you do they're definitely gonna be gone in eight years - can I get an address?"
"Sure," he says. "I just don't know if I'm still going to be living there in eight years." Regardless, he rattles one off.
Cam notes it. "Right, but it's a place to start. Okay, anything else you want before I go find someplace to build myself a tidy little base of operations?"
John hesitates, but then shakes his head.
"I'm good," the Joker says sunnily. "Have fun."
"All right. Oh, one more question - where, loosely, am I? I have never heard of Gotham, I don't think my world had one, it's a big enough city I would've heard of it. Is this America? Are we in a state?"
"Yeah, New York."
And Cam heads for the window.
"Go for it," says the Joker, waving.
And he hops into it and flies away.