Well, I should say that it's easy to do a passable job, hard to do a great job.
In the old days of wired phones, there were some great options. Tele-Q boxes (a box that generates the right voltage to ring the phone - just plug the cable into the box and ring away!) worked well, as did specials/recordings and hiding bells in secret places. Even with cordless phones, there were plenty of ways to get a truly great effect - tele-q boxes (attached to the base station) and specials both worked well.
Mobile phones, on the other hand, are much trickier. The tele-q option doesn't work, because there is no base station. Hiding specials doesn't work very well either, because people carry their phones with them, making the sourcing very tough. A couple of recent productions I know of used an iphone app called Airfoil, which streams audio directly to your phone, but in my experiments with it, there's a variable lag of 1-3 seconds between when I tell the sound to play and when it actually plays. That lag is unacceptable in a theatre environment.
Plus, if you want get picky, the sound of a phone ringing varies dramatically as you pull the phone out of your pocket, purse, etc. That's a complex change that is tough to replicate.
So, sound designers have been poking around this issue for years, and each designer has a couple of solutions that work well enough for them. I tend to use specials, and if the situation exists, I'll build two rings, an in-pocket ring and an out-pocket ring. It's good, but not great.
But that may soon change. We may soon have something awesome that will change our lives! This part of our lives, anyhow... stay posted!