My experience with ITP has been virtually flawless from the beginning. I ordered online and the supplied hardware arrived three days later. Set up was simple, consisting mainly of connecting cables according to instructions.
After I did so and fired everyhing up, I was able to make calls immediately. Porting my landline number also went smoothly. The instructions on the web site and LOA form were clear enough to me. I emailed the documentation on Saturday and received a confirmation email on Monday. Two and a half weeks later I received an email telling me the porting process was finished, and another reminding me to update my e911 information. End of story. There were no problems or noticeable interruption in service during the change-over.
We've used the service for two months, which probably isn't long enough to give a really complete picture, but during that time the sound quality on calls has been consistently high, and we have not experienced any service outage, dropped calls, or any artifacts like echos, time lags, or noise that I've heard can occur w/ voip. The worst that I have noticed is occasional voice breakup that lasts for a few seconds. This is pretty uncommon, and I suspect it has more to do w/ my internet connection than anything ITP is doing. At the beginning I was a bit nervous about switching over to voip, and I kept asking my wife about calls she'd made, and so far all she has ever said is, "It was fine, it sounded like a phone." That pretty much sums it up.
I've contacted phone support a few times for information, and each time I've waited for no more than a minute or two on hold, the support person has spoken english well, and I've received useful information. The web site is pretty basic and kind of cheesy looking, but it is functional and always available. The standard service features are comparable to or better than others I've look into, and every one that I've tested has worked. There is an odd quirk to the web site in that once you are logged on you can't reach the pages w/ faqs and description of the various service features, so if you want to test out some feature you're unfamiliar with, you have to actually log off to get to a description of the feature you're configuring. I consider this a pretty minor flaw. All in all, I have no problem w/ the customer service I've experienced.
If there's anything I don't like, it's the marketing tactics, particularly the way they advertise their prices. The basic service we signed up for is advertised as $9.99/mo (as of 4/2009), but there is an additional "ITP line fee" of $1.99 each month. So, in essence the service is $11.98/mo plus an e911 recovery fee and a "Federal universal service fee" that seem pretty typical for this type of service, and total $3.09/mo. On the other hand, when I called customer support they cheerfully told me the total cost (and also warned me there could be additional local taxes, which didn't apply in my case), but one shouldn't have to do this to find out the 1 cost of a product.
There seems to be sort of a subculture that likes to tinker w/ their voip equipment and/or jump around from service to service, and I wouldn't recommend this service to those individuals - I don't believe there is any "BYOD" option, there are no user forums, etc. However, for anyone who wants a service that is simple and, as they say, "just works", I would say that ITP is a service that you should consider seriously.