SIP troubleshooting
Registration failures, 401 and 403 responses, one-way audio, NAT and SIP ALG problems, and how to narrow each one down.
- Troubleshooting
How NAT Affects SIP and RTP
SIP writes IP addresses inside its messages, so network address translation breaks it. This article explains rport, symmetric RTP, STUN, ICE, TURN, keep-alives and SIP outbound, and what a softphone can and cannot do about NAT on its own.
9 November 2026 · 6 min read
- Troubleshooting
How to Troubleshoot No Audio in a SIP Call
When a SIP call connects but both sides hear silence, the usual causes are a codec mismatch, RTP blocked in both directions, a missing microphone permission, the wrong audio device, mute, an SRTP mismatch or a Bluetooth route that captured the audio. Work through them in order.
9 November 2026 · 5 min read
- Troubleshooting
How to Fix One-Way Audio in VoIP Calls
One-way audio means the call connected but RTP flows in only one direction, almost always because a private IP address ended up in the SDP. Fix it with STUN and ICE on the phone, NAT settings on the PBX, symmetric RTP and an open RTP port range.
9 November 2026 · 5 min read
- Troubleshooting
Why Can I Register but Not Make Calls?
Registration proves your credentials; calling tests the dialplan, outbound routes, dial format, codecs, trunks and caller ID. Learn to read 404, 403, 488 and 503 replies and find the setting that blocks your outbound call.
8 November 2026 · 5 min read
- Troubleshooting
How to Fix SIP 401 Unauthorized
A single SIP 401 Unauthorized is normal digest authentication. Repeated 401 replies mean the password, auth username or realm is wrong, or the account is locked. Here is how to tell the two apart and fix it.
9 November 2026 · 5 min read
- Troubleshooting
How to Fix SIP 403 Forbidden
SIP 403 Forbidden means the server understood the request but refuses it. Causes range from wrong credentials and disabled extensions to IP bans, fail2ban, wrong domains and outbound calls the dialplan does not permit.
9 November 2026 · 5 min read
- Troubleshooting
How to Fix SIP 408 Request Timeout
A SIP 408 or a plain registration timeout means the server never answered. Check the address, port and transport, UDP and TLS port blocks, SIP ALG, DNS SRV records, the server itself and the Local Network permission on Apple devices.
9 November 2026 · 5 min read
- Troubleshooting
What Is SIP ALG and Should You Disable It?
SIP ALG is a router feature that rewrites addresses inside SIP packets to help with NAT. It often does more harm than good, causing dropped registrations, one-way audio and missed calls. Here is how to recognise it, why it breaks, and how to turn it off.
9 November 2026 · 5 min read
- Troubleshooting
Why Does SIP Registration Keep Failing?
A systematic checklist for SIP registration failures: credentials, transport and port, DNS, NAT keep-alives, registration expiry, TLS certificate validity, contact limits, IP bans, Wi-Fi versus mobile networks and IPv6.
9 November 2026 · 5 min read
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