Identify the Symptom First
Different quality problems have different root causes. Treating them generically wastes time.
| Symptom | Most Likely Cause |
|---|---|
| Choppy or robotic audio | Jitter or high latency |
| Words cutting out | Packet loss |
| One-way audio | NAT traversal and firewall |
| Echo | Acoustic echo cancellation issue |
| Call drops after 30 seconds | SIP timeout and ALG enabled |
Step 1: Measure Before You Fix
Run a VoIP quality test from the affected device or network segment. Target values:
| Metric | Good | Acceptable | Problem |
|---|---|---|---|
| Latency | under 50ms | 50 to 150ms | over 150ms |
| Jitter | under 10ms | 10 to 30ms | over 30ms |
| Packet loss | 0% | under 0.5% | over 1% |
Fix: Jitter
- Enable QoS on firewall and all switches
- Mark VoIP traffic as DSCP EF (Expedited Forwarding)
- Prioritise VLAN 50 (or equivalent voice VLAN) in switch config
- Increase jitter buffer on soft PBX if available
Fix: Packet Loss
- Check switch port error counters for CRC and frame errors
- Replace bad patch cable — always test before blaming software
- Check for interface duplex mismatch
- Check ISP stats for upstream packet loss
Fix: One-Way Audio
- Disable SIP ALG on router and firewall (most common cause)
- Open required RTP UDP port range in firewall
- Configure correct external IP on PBX if behind NAT
Fix: Calls Dropping After 30 Seconds
This is almost always SIP ALG rewriting SIP headers. Disable SIP ALG on your firewall first.
Fix: Echo
- Enable echo cancellation on PBX
- Reduce handset speaker volume slightly
- Confirm correct codec (G.711 preferred over G.729 for LAN)
*Related: IP Telephony Dubai | Networking and Security Dubai*
Tags
VoIPCall QualityTroubleshootingNetworkingHow-To
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