Started w/Vonage back in 2003. Dumped them due to pricing, a few reliability problems at key moments, and adapter issues. Broadvoice...ugh..don't get me started.

For residential or small business "plug & play" for 1-2 phone lines, I'd recommend Viatalk. Practically an identical service to Vonage but $12/mo with lower international rates. More features as well. Not 100% reliable, but probably sufficient for most people. And they'll let you use any equipment you want with their service. Want to use Viatalk with your ATA and softphone on your laptop with the same number? no problem.

BUT I wouldn't use Viatalk with an IP PBX. They're set up more to provide traditional replacement for a POTS (plain old telephone service) line than PRI/T1 replacement. 2 outgoing & 2 incoming channels per DID, but their service ignores the CallerID your IP PBX is sending and replaces it with what the channel's registered as with their service.

Skype's not bad, especially the newer versions. Sound & video quality's quite good and it does tunnel nicely through most firewalls. There's a setting inside it you can enable if you don't want it using your bandwidth for other people's calls. Tweaked out properly, it's just a notch below broadcast TV quality. I'm not as much a fan of their SkypeOut/SkypeIn service, but that's only because we have MUCH better options through our phone system (below). SkypeOut/SkypeIn use the G.729 audio codec which is closer to cell phone quality than landline and it's 2-3x more expensive compared to wholesale VoIP providers. Not that we're talking huge #'s compared to traditional telcos here. It's still a bargain compared to the telcos.

If you've got some tech savvy, go wholesale VoIP. Aretta.com is cheap if you're not going to use it much or your # of simultaneous calls varies drastically. We use them for our conference bridge and as a backup to our main service. Bandwidth.com is good if you need absolutely bulletproof service and are willing to pay for it. I know a couple of people who use Vitelity, CallCentric, and Voicepulse and have been happy with them. I've not tried them personally.

We currently have an Asterisk 1.4 PBX at the office, feeding 3 other offices in 3 other states. We also let employees take VoIP phones home. Some of them have chosen to port their home #'s over to us as well and charge them $5/mo for phone service, all-you-can-eat local & US/Canada LD. Actually costs me ~$2.50/mo to maintain DID for their home #, with the extra $2.50 going to pay for the bandwidth/minutes used/staff support hours. Not a bad deal.

By the time all is said & done, we've got unlimited US/Canada calls, paying $0.0125 to call London and my phone bill dropped from $900/mo to ~$150/mo, including all of our conference calls. The Cisco & Polycom phones are dirt cheap ($20-$150) on the used market (eBay) and superior to the Samsungs they replaced. Then there's the features. The vast majority of our employees now work from home at some point during the week, people routinely take their "extension" phone on the road. At this point the only one who truly knows whether people are in the office or not is the receptionist. As I write this I'm watching roseate spoonbills along the Gulf of Mexico, office phone sitting on the desk next to the laptop. My clients (and coworkers) are none the wiser. VoIP's a beautiful thing.