SS7 — Signaling System 7
What is SS7?
SS7 (Signaling System 7) is the legacy signaling protocol stack used by the PSTN (Public Switched Telephone Network) for call setup and teardown, SMS routing, and network intelligence functions like roaming and fraud prevention. Most calls still traverse SS7 networks at some point, even when both endpoints use VoIP, because calls ultimately reach traditional landlines or mobile carriers through the PSTN's SS7 infrastructure. Modern VoIP and wholesale voice carriers interface with SS7 networks through a softswitch or media gateway to reach traditional phone numbers.
More detail
SS7 uses out-of-band signaling, where call control messages travel on a separate network path from the voice media itself, allowing the PSTN to route, authenticate, and track calls across multiple carriers and countries.
While VoIP has modernized internal call handling with SIP, the PSTN's SS7 network remains the global standard for carrier-to-carrier signaling and is essential for roaming, number validation, and SMS delivery across traditional telecom infrastructure.
Frequently asked
- Does my VoIP call use SS7?
- Not directly. Your VoIP system uses SIP for call signaling, but when your call needs to reach a traditional landline or mobile phone, a carrier gateway translates it to SS7 to traverse the PSTN, so your call may touch SS7 even if your endpoint doesn't speak it.
- Why do softswitches and carriers still care about SS7?
- Because the PSTN still carries the majority of phone calls worldwide, and SS7 is how carriers route calls, authenticate caller ID, enable roaming, and perform HLR (Home Location Register) lookups — all critical functions that don't exist in pure VoIP networks alone.
See also
Build it on Orbit
Voice, messaging, email, video, and AI agents on one platform and one pay-as-you-go bill. Start free — no credit card required.