...land-line phones become a thing of the past?
I say 30 years for most countries, maybe another 10 for the poorer ones*. Though if recent history has taught us anything, it's going to be even less than that.
It's frightening to see how rapidly and how significantly the world is changing. A mere decade ago mobile phones were a rare and exotic gadget. Now, everyone has one, even in the Third World, and it won't be long before they come standard with a camera, WiFi, full colour screen and mp3/video playback. Probably act as a credit card before long, and GPS to boot.
*and by this I mean compltetely gone. A gradual shift away from fixed phones will probably happen much sooner. In fact, it's already in progress.