Switching from Phone to Voice-Over-IP?

An anonymous reader asks: "I am fed up with the telcos. My last phone bill for one line was over $100 _without_ the $45 for ADSL, so i'm looking to cut as many costs as I can. I've compared my current Voice + Internet solution to one consisting of Voice-over-IP and Cable Internet and it looks like I would save over $50 a month by throwing away my land-line phone and switching to Cable/VoIP. I'm new to the whole changing-from-POTS-to-VoIP process though, so what are some of the 'gotchas' involved with switching? Can I keep my existing number? Will calls suck my bandwidth dry? And most importantly, do any of these VoIP providers work with or support Linux?"