VoIP originally was sold as the answer to the next wave of cost savings for consumers and especially for small business. Business phone lines have always cost more than residential lines because, in theory, businesses use the phone line more often. (I guess, if you have 2 teenagers then that equals 1 small business).

Now that businesses are looking for ways to cut costs, analysts are predicting a rise in VoIP sales. Maybe. Define VoIP.  Hosted PBX certainly offers an attractive ROI, but to take full advantage of it, there is an upfront CAPEX (capital expenditure) that includes: IP Phones, cabling, POE switch, and some form of IAD or QOS Router like an Edgewater.   A lease could alleviate some of this, but Voxilla agrees with me on the CAPEX question.

If we are talking about the myriad apps that offer VoIP on the cell phone, I have to ask, why? These apps don't work on every handset. These apps either use up minutes (on callbacks) or data. It seems that it would be cheaper to bulk up the minutes than go through the trouble. But Gary Kim writes that the small business segment will move mainly to cellular IF the iPhone can become like the desk phone. I guess people like to ask, "Huh? What? you cut out there." Or more likely they aren't paying attention any way so don't care about call quality, dropped calls, or dead batteries.

VoIP mainly makes sense when there is a lot of inter-branch calling. Or when there is a virtual office, tele-workers, or other location varied need.

While SIP Trunking is the new buzz word, it is sold primarily as a PRI replacement, which makes no sense to me.  TDM and PRI are tried and true. You might save 10%, but so what? SIP Trunking isn't a standard so it may or may not work with your PBX IP card, depending on carrier and implementation. And how much cheaper do you think long distance will be on a SIP Trunk versus a PRI?

If the SIP Trunk is a means to extend the life of a IP-PBX as well as add SIP overlay features, that would be a better offer, but that isn't the "I'll save you 10%" deal that most folks tout. 

Overall, cellular will probably win for the mobility.