Broadband phones are becoming more mainstream. There are articles whether broadband phone service reached a tipping point or not. What is clear, though, is that phone service delivered over the public internet is the future of telecommunications.
Let's take a step back and examine what the uses are for broadband phone service. More specifically, the technology behind it. Broadband phone service is utilized by a technology called Voice Over IP (Internet Protocol), or VoIP. It utilizes the bandwidth (the "speed" in layman's terms) to send voice packets. These packets are sent like any other data (ie email, web browsing etc.). When they reach their destination the data packets are reconstructed and converted into voice!
All of this happens in a fraction of a second but many issues can arise to degrade the broadband phone service quality. A slow connection, bad phone, over-utilized bandwidth and a bad service provider can all degrade the quality of the phone call.
While broadband phone service uses VoIP not all VoIP is using this service, at least in the traditional sense. Companies with multiple office can communicate using VoIP over direct connections or over the public Internet. Even though between the remote offices they are communicating over VoIP when a call is made outside the company they can still use regular phone service (PSTN or T-1 service). This allows them to take advantage of Voice Over IP without the detriments of this service.
There are usually significant cost savings by using broadband phone service that can alleviate the negative effects of service quality. For the general consumer this might be an acceptable trade-off. But a commercial enterprise might not want to accept this risk.
Another issue that arises with broadband phone service providers is their longevity. Since it's relatively inexpensive to start a VoIP company you should be extra cautious about the background of the company before you commit your number. Many consumers have been burnt by using new companies who offered too good to be true deals. Sometimes when it's "too good to be true" it IS too good to be true. Buyer beware.
There are many cool features that broadband phone companies provide. For example, many of them email you your voice-mails. You can open your email and listen to the audio attachment on your computer. Some also offer a voice menu system that you can change, when you want, on their website.
As the technology matures and the quality gets better broadband phone service will be the norm and old legacy phones will be just that, a legacy from the past.