Most people's savings are coming from two things:
1) Going from a landline to VOIP.
2) Going from regular pricing at a mix of providers to promotional pricing for their first year at the new provider.
Which means you have to consider two things: if I'm going VOIP anyway, am I getting the lowest pricing from my cable company, or could I save more with getting their internet and someone else's VOIP? And, am I going to be saving this much after my first year?
If you're a speed demon, you also have to consider the quality of your internet service. Around here Verizon advertises DSL for 14.95 a month, and in very small print advertises that you get speeds of up to 768kbps. For me, and I realize I am probably in the minority here, I would rather pay three times that for 10mbps through my cable provider. (And, before you ask, this is not a theoretical 10mbps--I hit 1meg/s easily off of a fast source.)
Also, consider some of the problems with VOIP before you sign up. My cable provider's VOIP cannot accept collect calls and cannot make 10-10 calls. It cannot support medic alert systems or life alert systems. If the cable goes out, your phone goes out. If the power goes out and you haven't sprung for the $50 battery backup (which provides 8-10 hours of standby time and ~6 hours of talk time), your phone goes out.
If all three are on one bill and money becomes tight, you cannot let the cable or internet bills go and just pay your phone. If you go into collections, all three are getting shut off. If you want to remove the internet, your phone is gone too. If you remove the TV, your promo pricing is suddenly back to full price.
No tax on the phone bill is very, very nice.
And, when you call the company to get pricing on what happens after the first year, don't talk to the sales department--they know exactly how to bend the truth to get you to sign up, and conveniently forget to include things like tax, equipment rentals, and other things when you sign up. The customer service department is used to dealing with the lies from other departments all day, and will tell you exactly what is going to happen during and after that first year.