Pillar 2 · Fixes
VoIP Troubleshooting & Fundamentals
Something is wrong with the phones. This pillar starts where every ticket starts — with the symptom a user can describe — and works down to root cause with captures, codes and measurements, the way a VoIP engineer actually diagnoses.
The method behind every guide here is the same: define the symptom precisely, capture the traffic, decide whether signaling (SIP) or media (RTP) is failing, isolate one variable at a time, fix, and verify with the same capture that proved the fault. If you are new to that workflow, read The Troubleshooting Method and Reading a SIP Ladder Diagram first — every other guide assumes them.
Fix by symptom
One-Way Audio
One side hears, the other hears silence. Almost always NAT, firewalling or a mismatched RTP range.
No Audio at All
Calls connect and both sides hear nothing. Work the media path from the capture outward.
Choppy Audio and Jitter
Garbled, robotic, underwater voice is a measurement problem: jitter buffers and congested queues.
Echo on Calls
Hearing yourself twice has acoustic, hybrid and network-delay causes — each with a different fix.
Robotic or Metallic Voice
Packet loss concealment and codec transcoding artifacts, and how to prove which one it is.
Calls Dropping Mid-Call
Random drops point at the network path, power or a failover event. A capture tells you who hung up.
Drops at Exactly 15 or 30 Minutes
A call dying on a timer implicates session timers, re-INVITEs and UDP NAT timeouts.
Phones Not Registering
REGISTER gets no reply, a 401 loop or a 403. Credentials, transport and NAT, in that order.
Phone Keeps Rebooting
PoE budgets, DHCP loops and provisioning storms behind the phone that restarts all day.
Calls Go Straight to Voicemail
DND flags, unregistered contacts and stale registrations that send callers to voicemail instantly.
Fast Busy Signal
Reorder tone means the call was rejected early. The SIP response code in the capture names the reason.
Long Delay Before Ringing
Seconds of silence after dialing: DNS, trunk hunting and interdigit timers, measured step by step.
Wrong Caller ID
Your calls show the wrong name or number: CNAM, STIR/SHAKEN attestation and trunk settings.
Fax Over VoIP and T.38
Why fax fails on compressed codecs and how T.38 fax relay is supposed to fix it.
DTMF Tones Not Working
IVR menus ignore your keypresses when RFC 2833, SIP INFO and inband DTMF get mismatched.
Quality Collapses at Peak Hours
Fine at 9am, awful at noon: congestion, oversubscribed circuits and missing QoS policy.
Static, Hum and Crackling
Analog tails, grounding, handset cords and packet loss each make a different noise. Identify yours.
Understand the fundamentals
How VoIP Works
The whole system in one guide: endpoints, SIP signaling, RTP media, and where the PSTN meets IP.
What Is SIP?
Requests, responses, dialogs and transactions — the protocol underneath almost every modern call.
RTP Explained
Sequence numbers, timestamps and jitter: how the voice itself travels, and how RTCP reports on it.
The Troubleshooting Method
The layered workflow professionals use: define the symptom, capture, isolate, fix, verify.
Reading a SIP Ladder Diagram
Follow one call through a capture, message by message — the core skill behind every fix here.
Wireshark for VoIP
Capture filters, RTP stream analysis and the telephony tools that turn packets into answers.
NAT Traversal Explained
Why NAT mangles SIP, and how symmetric RTP, SBCs and STUN each solve a piece of it.
SIP ALG Explained
The router feature that rewrites SIP packets — and breaks registrations and audio when it guesses wrong.
Firewall Ports Checklist
Every port a VoIP deployment needs, by function, with a checklist you can hand to a network admin.
Codecs Compared: G.711, G.729, Opus
Bandwidth, quality and CPU trade-offs for the codecs you will actually meet, with the wire-rate math.
QoS and DSCP for VoIP
Marking, queuing and policing: how networks protect voice, and the DSCP values that matter (EF, CS3).
MOS Score Explained
What a Mean Opinion Score of 4.2 vs 3.1 actually means, how it is estimated, and its limits.
Jitter, Latency, Packet Loss
The three numbers that decide call quality, the thresholds that matter, and how they interact.
Packet Loss Diagnosis Workflow
From 'calls sound bad' to the exact hop dropping packets, using captures at two points.
What Is an SBC?
Session border controllers: NAT fixing, security, transcoding and why carriers put one at every edge.
STUN, TURN and ICE
How WebRTC-style traversal finds a working media path — increasingly relevant to desk phones too.
SIP Session Timers
RFC 4028 refresher mechanics — and why they are the prime suspect in timed call drops.
TLS and SRTP Encryption
Encrypting signaling and media: what it protects, what it costs in CPU, and where it breaks interop.
DNS SRV and NAPTR for VoIP
How phones and trunks find servers, fail over, and why a DNS hiccup looks like a phone outage.
Decode the error: SIP response codes
When a call fails, the network usually says why — in a three-digit SIP response code. Our SIP response code library explains each common code: what generated it, what it rules out, and the fixes in the order to try them.
SIP 401 & 407
Authentication challenges: digest auth, registration loops and credential failures.
SIP 403 Forbidden
The server understood and refused: ACLs, disabled accounts and trunk permissions.
SIP 408 Request Timeout
No answer from the far end: unreachable endpoints, NAT pinholes and dead trunks.
SIP 480 Temporarily Unavailable
The endpoint is registered-adjacent but cannot take the call: DND and unregistered contacts.
SIP 486 Busy Here
A genuine busy from the endpoint itself — and how it differs from network congestion codes.
SIP 488 Not Acceptable Here
Negotiation failed: codec lists, SDP mismatches and media settings the far end refuses.
SIP 500 Server Internal Error
The server failed itself: crashes, overload and misconfiguration on the provider side.
SIP 503 Service Unavailable
Overload, maintenance or a dead upstream route — and what Retry-After tells you.
SIP 603 Decline
A global refusal: the user (or their policy) declined everywhere, not just on one device.
A note on scope
These guides cover the faults small and mid-sized businesses meet on hosted VoIP, SIP trunks and on-premise systems. They are educational: they will help you diagnose, speak precisely with your provider, and fix the large share of problems that live in configuration and the local network. Carrier-core faults and life-safety configuration still belong with your provider and a qualified engineer — each guide says plainly when you have reached that line.