In its early days, VoIP was touted for many reasons, but quality was seldom one of them. VoIP was traditionally seen as a riskier technology, with near-certain problems with quality and reliability. Land lines (especially digital circuits such as PRI) were so reliable that one could reasonably expect decades of trouble-free service from them.
Carriers would put their brightest and best on the PRI side of the business; that’s where all the big customers were, and that’s where the money was to be made. Today, the old-school technical people are retiring, and the newcomers to the industry have little or no interest in obsolete technologies such as PRI.
Today, I find I can no longer recommend PRI service as the best choice for on-site trunking. The reason is not so much technical as it is cultural: Younger, career-minded people rising through the ranks in this industry cut their teeth on IP-based technologies, and old-school legacy voice is neither interesting to them, nor in many cases even comprehensible.
In the very recent past, the best way to ensure trouble free PSTN connectivity was PRI. Today, that is no longer a safe assumption. As the carriers work to cut costs, and as the decision-makers in these organizations are people with a career history of learning and working with VoIP, the older, more expensive technologies have become little more than peripherals, reluctantly installed to deliver backwards-compatibility. In many cases if you order a PRI circut, what you’ll find is delivered is a SIP-based gateway, providing VoIP the whole last mile to your equipment room, with a 6 foot cable of PRI into your phone system. If your PBX is VoIP-capable, you might as well cut out the 6 feet of PRI and go SIP the whole way.
Note that this is more of a trend that I’ve been seeing rather than a hard rule that applies in all situations. Still, I have seen increasing problems with PRI, and decreasing problems with VoIP (not just with the technology, but also with the people involved in providing it). The tipping point was already several years ago, and whereas five years ago I would have always recommended PRI, today, I very seldom do. My experience is that the best and brightest people on the carrier side of things are working in the VoIP side of the house, and the PRI side of the business is mostly old timers nearing retirement, or folks who do not seem to understand the technology very well.
The bottom line is that while a PRI circuit was always a very low-risk way to deliver trunking to your premises, that is no longer a certainty. The best minds are working in VoIP now; from product development, through solution engineering, to installation and support.