Glossary Core concepts
What is PSTN?
Also called public switched telephone network, landline network
The PSTN, or Public Switched Telephone Network, is the worldwide network of telephone systems that lets any phone number reach any other. It began as analogue circuits and manual switchboards and is now overwhelmingly digital, but the term still means the same thing: the public network, as opposed to a private system inside one organisation.
Why it still matters if everything is IP
Even a business running entirely on VoIP still needs to reach and be reached by the PSTN, because that is where its customers are. The point at which your IP calls hand off to the public network is where a carrier is involved, and it is the reason phone numbers, regulation and per-minute charges exist at all. Internal calls between two extensions on the same system never touch it.
The move off legacy circuits
Carriers worldwide have been retiring analogue and ISDN services for years, which is what pushed most businesses toward SIP. The practical effect is that "keep the old lines" stopped being an option on its own timetable; the copper is going away whether or not a given business was ready to plan for it.
Interconnection, and why it shows up on your bill
When your carrier hands a call to another carrier to complete it, money changes hands between them. Those interconnection and termination arrangements are why per-minute rates exist, why calls to some destinations cost more than others, and why international rates vary so widely by country. It is also why a provider that owns network infrastructure can price differently from one reselling somebody else's capacity: they are on a different side of that settlement.