+1 to this, that's for sure.
VOIP, cell phones, etc - are simply not reliable enough if some important portion of your job depends on you being on a conference call. I spend $20ish/month on a regular, low-frills analog phone line because it always works. I know when I can't understand someone on a call, it's because of a problem with their phone, and not mine.
That leads me to another point, if you do spend a good amount of time on conference calls, invest in a good desktop phone, like a Polycom. They're expensive (you get to write it off as an office expense, in the States, at least), but just another one of those things that lets you communicate clearly and effectively and not be the "that guy/gal" that nobody can ever understand over the echo.
Definitely also have an internet access backup plan, even if it's just having your phone ready to tether or knowing how fast you can get to Starbucks. I've found that there's some slack for the occasional ISP issue, but you have to be ready for that "emergency" where you have to be connected, *now*.