Time to Ringing
Understand the Time to Ringing metric — how CallMeter measures the delay from SIP INVITE to 180 Ringing, and what it reveals about call routing and remote endpoint responsiveness.
Time to Ringing measures the delay between sending a SIP INVITE and receiving a 180 Ringing response, in milliseconds. The 180 Ringing indicates that the remote phone has been located and is now ringing. This is the moment the caller's perception shifts from "connecting" to "ringing."
Think of it as the time between dialing a phone number and hearing the first ring. Everyone has an intuitive sense for how long this should take — a second or two feels normal, five seconds feels slow, and ten seconds makes you wonder if the call is going through at all.
One-shot metric
Time to Ringing is captured once per call, not as a time-series. It appears in the Call Timing Overview cards on the endpoint detail page.
How It Works
When CallMeter sends a SIP INVITE, the call traverses the signaling path: SIP proxies, registrars, and Session Border Controllers route the call to the destination endpoint. When the destination device accepts the INVITE and starts alerting the user, it sends a 180 Ringing response back through the same chain.
The time-to-ringing value is: ringing_at - invite_sent_at
This encompasses the entire signaling chain:
- INVITE travels from caller to first SIP proxy
- Proxy authenticates and routes the call
- Call may traverse multiple proxies, gateways, or carriers
- Destination endpoint receives the INVITE
- Destination sends 180 Ringing back through the chain
Why It Matters
Time to Ringing is a key customer experience metric. It measures the responsiveness of the entire call setup chain — not just one server, but every hop from the caller to the callee.
For enterprise VoIP testing:
- Carrier comparison — Different SIP trunk providers route calls differently. Time to Ringing reveals which carrier provides the most responsive path.
- Route optimization — Slow ringing times on specific destinations may indicate suboptimal routing that can be improved.
- International calls — Calls to international destinations typically have longer ringing times due to additional routing hops.
- Infrastructure changes — Compare before and after infrastructure updates to detect regressions.
Common Causes of Slow Time to Ringing
| Cause | Explanation |
|---|---|
| Multi-hop routing | Call traversing many SIP proxies or gateways adds cumulative latency |
| Carrier processing | SIP trunk provider performing number translation, fraud checks, or load balancing |
| DNS lookups at each hop | Each SIP proxy resolving the next hop via DNS |
| Authentication delays | Challenge/response authentication adding round trips |
| Remote endpoint slow to alert | Destination device taking time to process the INVITE and begin ringing |
| PSTN interworking | Calls to traditional phone networks involve SIP-to-ISUP translation |
How to Fix It
- Compare with Time to Trying — If Time to Trying is fast but Time to Ringing is slow, the delay is in the downstream routing chain, not your immediate infrastructure.
- Test different carriers — Route the same call through different SIP trunk providers to compare their routing performance.
- Review call routing rules — Simplify routing logic where possible. Each routing decision adds processing time.
- Check for unnecessary hops — Audit the signaling path for SIP proxies or SBCs that are traversed but not needed.
- Monitor specific destinations — Slow ringing to specific number ranges may indicate a problem with a particular carrier route.
Related Metrics
- Time to Trying — First-hop responsiveness; compare to isolate where the delay occurs
- Post-Dial Delay (PDD) — Similar measurement but focused on when the caller perceives ringing
- Call Setup Time — Total time from INVITE to answer
- Final Status Code — If 180 Ringing never arrives, reveals what happened instead
RFC Reference
The 180 Ringing provisional response is defined in RFC 3261 (SIP: Session Initiation Protocol), Section 21.1.1. The UAS generates 180 Ringing when it begins alerting the user, as described in Section 13.3.1.1.
Time to Trying
Understand the Time to Trying metric — how CallMeter measures the delay from SIP INVITE to 100 Trying, and what slow response times reveal about server health.
Post-Dial Delay (PDD)
Understand Post-Dial Delay — the industry-standard metric for measuring the time from INVITE to first ringback, thresholds, and why PDD is the most important call setup KPI.