Yeah, this sort of thing worked for me maybe in 2006-2009. At least in my experience. Some very fun dates or short term relationships came from me hanging out with coworkers. Wal-mart, no less. Haha!
Today though, I've got to agree with the idea that it is generally not a good idea to pursue a co-worker. Chances are, especially if this kind of banter has lasted a year, that she is less likely to be into you "that way." I mean, you never know. If you've been proximally around each other and neither has made a move (lack of it on her end probably means she isn't into you, you're not her "type" OR doesn't think you'd be receptive to her flirting with you. Lack of it on your end, until now trying to get your mojo up for her digits, communicates to her in undertones that you weren't interested in her "like that." To which she could be wondering "Why is he asking now?") there's still the possibility of meeting up outside of work and feeling it out. She could just be shy, right?
I wouldn't worry about the number right now. If ya'll seem to strike it off well, great. Now you have an excuse for having her number. This could just be me, but I think getting digits before doing anything else is something that is done if you are interested, but doubt you'll run into this person again. I think the best bet for getting with anyone at work are new co workers or if you are new on the job. You start clean. If you like her and think she's worth some effort, ok go for it.