The PSTN is a network of networks: incumbent operators, mobile networks, competitive carriers, and, increasingly, IP multimedia cores that still present a telephone interface at the edge. What unifies it is not a single switch technology. It is the public numbering plan (ITU-T E.164), interconnection obligations, and a signaling system that can set up a call from any connected access to any other.
Calling it 'legacy' is half right. Many last miles are still analog or TDM. Many cores are already IP (softswitches, IMS). From a business VoIP system's point of view the PSTN is the set of destinations that are phone numbers rather than SIP URIs, plus the gateways and policies required to reach them.
What the PSTN is
Historically the PSTN was circuit-switched: a call occupied a 64 kbit/s timeslot (DS0) from end to end, or an analog local loop into a Class 5 switch. Signaling moved from in-band tones to common-channel SS7. Access diversified into ISDN, cable telephony, mobile (GSM, CDMA, then LTE/5G), and wholesale VoIP. The service itself has not changed: a caller dials a number, the network rings the destination, and billing and lawful intercept still attach to that call.
National regulators treat PSTN interconnection as critical infrastructure. Numbering, emergency calling, and wholesale origination/termination rates are matters of public policy, not only engineering choices. That is one reason a pure internet app cannot fully replace 'the phone network' even when audio quality on the app is better.
Signaling: SS7 and ISUP
Signaling System No. 7 (SS7, ITU-T Q.700 series; ANSI variants in North America) is the common-channel signaling system that set up, tore down, and queried calls between traditional switches. ISUP (ISDN User Part) carries call-setup messages analogous to SIP INVITE and BYE. MAP and other application parts support mobile roaming and SMS on SS7, which is a separate security story from voice.
IP interconnects often replace ISUP with SIP (sometimes SIP-I / SIP-T that encapsulate ISUP for fidelity). The functions remain: calling number, called number, continuity, release codes. A SIP 404 and an ISUP cause code are different dialects of 'this number cannot be reached.'
Numbering: E.164
Every PSTN reachable line is identified in the international public numbering plan, ITU-T E.164: country code, national destination code, subscriber number. DIDs, local numbers, and toll-free numbers are allocations inside that plan (or national equivalents that map into it). Number porting moves a subscriber number between carriers without changing the digit string the public dials.
VoIP systems that want to be reachable from a mobile keypad publish E.164 numbers, not only sip: URIs. The URI can be used on-net. Off-net callers will not type it.
How VoIP interconnects with the PSTN
A media gateway or an IP peering session sits on the border, with SIP/RTP on one side and SS7 and PCM (or SIP/RTP to a carrier who still owns the TDM) on the other. At the wholesale layer, that border is sold as origination and termination over SIP trunks. Codec negotiation often collapses to G.711 at this edge because the PSTN's historic payload is 64 kbit/s PCM.
| Border function | VoIP side | PSTN side |
|---|---|---|
| Signaling | SIP (RFC 3261) | SS7 ISUP or SIP-I |
| Media | RTP, various codecs | G.711 PCM / TDM timeslots, or IP PCM |
| Identity | From/PAI headers, optional STIR/SHAKEN PASSporT | Calling party number, ANI, ISUP parameters |
| Addressing | sip: URI or tel: URI | E.164 digits |
| Business process | CPaaS or PBX dial plan | Carrier routing, LNP, interconnect contracts |
Why the PSTN is not going away
Copper loops are being retired in many countries. That is not the same as retiring the PSTN as a service. Mobile networks, IP voice, and remaining wireline all still terminate E.164 numbers. Emergency services (E911 and national equivalents) are built around that numbering and around regulated access to PSAPs. Banks, governments, and machine-to-machine alerting still dial numbers because they work from any handset without an app install.
What is going away in many markets is TDM as an access technology. What remains is interconnection, numbering, and a lowest-common-denominator audio path (narrowband PCM) that HD Voice can only avoid by staying on-net.
Key terms
- Class 5 switch
- The historic local exchange that served subscriber lines. Softswitches and IMS now play this role in many networks.
- Tandem / Class 4
- A transit switch between local exchanges. Least-cost routing in VoIP is the IP analogue.
- LNP
- Local Number Portability: the database lookup that finds the current carrier for a ported E.164 number.
- POI
- Point of interconnection: where two carriers hand off calls, now often Ethernet SIP instead of SS7 over T1/E1.