In episode 2 or 3 of Luke Cage, he is asking around town if anyone has seen a lad, showing a picture of him on a fairly large/newish Android smart phone. But Luke is struggling to find cash for rent, and is trying to live off the grid by working off the books for cash, and do so honestly (edit: or at least, according to his personal honor code), all of which would seem to make it tricky and a strange priority if it were his own phone. Having a photo of the lad on it seems a bit odd too. Was he borrowing Pop's cell phone, or was it just manifesting out of comic book handwave or advertiser suggestion?
These days, cell phones are ubiquitous in TV and movies, because they help speed the plot along so conveniently. Writers rarely bother to stop and explain where they came from, and they didn't do so here.
However, there's a couple of ways he might have one:
- He's only been working at Pop's for about 5 months. As some point before that (within the past year, at least), he owned a bar and made enough money to keep it open. While he lost his bar and home when Kilgrave forced him to blow up the bar, some smaller items, like his cell phone, might have been on his body and survived. This doesn't explain how he pays the bill, but maybe he's always had a prepaid one (since he's fugitive anyway).
- There are programs where low-income people can obtain cheap cell phones from social services. Having access to a phone is often important to finding and keeping a job, taking care of children, etc., so it's becoming considered less of a luxury item and more of a necessity.
- Or, the easiest answer, one of his friends or employers bought it for him. His jobs don't really seem high-paying enough to justify that, but some employers supply their employees with cheap cell phones to guarantee they can be on call all the time. Or Pops might just be done Luke a favor.
The most top of the line smart phone is only 600~700 bucks, bought straight out. Less or free with contact. Without a specific model, we can't assume it's the highest end phone. Even pre-paid service have high quality phones. And in 2016, a smart phone is as ubiquitous as a TV or car, even for someone paycheck to paycheck on two jobs struggling to pay rent. You can't get work or around without one in modern day New York.
It's simply just likely that he bought a decent phone, out of pure necessity. Luke also has nice clothes. He isn't rocking tattered shirts or jeans.
Hell, go walk around New York City, even some homeless or children have quality smart phones.