Support Center / Troubleshooting
One-Way Audio on VoIP Calls — Causes and Fixes
One-way audio — you hear them, they can't hear you, or the reverse — is almost never the phone. Voice media (RTP) flows separately from call signaling, and when a router mangles or blocks the media path in one direction, the call connects normally but audio flows one way. The usual culprit is SIP ALG.
Rule out the trivial first
- Mute — is the mic muted? (Every IT person has been burned by this.)
- Headset phones: a dead or half-seated headset produces "silent mic" symptoms; test with the handset.
- One caller only, or every call? One caller points at their end.
The network causes
- SIP ALG enabled on the router. Disable it and retest — this fixes most persistent one-way audio. See network requirements.
- Firewall blocking RTP. Ensure outbound UDP 10000–20000 is permitted and that return traffic is allowed (stateful rules handle this; strict asymmetric rules don't).
- Double NAT — two routers between phones and internet (common when an ISP modem/router feeds a second office router). Bridge one of them.
Pattern diagnosis
Symptom | Likely layer |
|---|---|
Every call, both directions dead | RTP fully blocked — firewall |
Every call, one direction | SIP ALG or NAT rewrite |
Intermittent, degrading audio | Congestion/jitter — QoS, not blocking |
One site only | That site's router; compare against a working site |
Escalating
Capture: extension, time of a failing call, direction of silence, and whether it affects one site or all. The support team can inspect that call's media path directly from the call detail records.
Related articles
Last reviewed August 6, 2026. Still stuck? Contact Signal Support from your portal — include what you expected, what happened instead, and one example phone number or extension.