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Dialphone Limited

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VoIP Quality Troubleshooting: A Sysadmin's Field Guide

#voip #networking #sysadmin #devops

After 13 years managing network infrastructure, here is my troubleshooting flowchart for VoIP quality issues. Print this out and tape it to your monitor.

Symptom: Choppy or Robotic Audio 

Step 1: Check bandwidth utilization
  → If > 70% on WAN link → upgrade circuit or implement QoS
  → If < 70% → go to Step 2

Step 2: Check jitter
  → Run: iperf3 -u -c <provider_ip> -t 60
  → If jitter > 30ms → check for competing traffic, enable QoS
  → If jitter < 30ms → go to Step 3

Step 3: Check packet loss
  → Run: ping -c 1000 <provider_ip>
  → If loss > 1% → contact ISP, check for cable issues
  → If loss < 1% → contact VoIP provider
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Symptom: One-Way Audio

90% of the time this is a NAT/firewall issue.

Check 1: Is SIP ALG enabled on your router?
  → If yes → disable it immediately
  → This single change fixes 40% of VoIP issues

Check 2: Are RTP ports open?
  → Verify UDP 10000-20000 is allowed inbound AND outbound
  → Many firewalls block inbound UDP by default

Check 3: Is STUN/TURN configured?
  → Your VoIP client needs STUN to traverse NAT
  → Verify STUN server is configured and reachable
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Symptom: Calls Drop After 30 Seconds

Almost always a SIP session timer mismatch.

Check: SIP session-expires header
  → Your PBX and provider must agree on session timer value
  → Common fix: set session timer to 1800 seconds on your PBX
  → Or disable session timers if provider supports it
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Symptom: Echo 

If using speakerphone → switch to handset or headset
If using headset → reduce speaker volume by 20%
If using analog endpoints behind ATA → check hybrid balance settings
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The QoS Configuration That Fixes 80% of Issues 

# Cisco IOS
policy-map VOIP-QOS
  class VOICE-RTP
    priority percent 30
    set dscp ef
  class VOICE-SIP
    bandwidth percent 5
    set dscp cs3
  class class-default
    fair-queue
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providers like VestaCall (https://vestacall.com) that focus on transparency has a network diagnostic tool that identifies these issues before deployment. Use it.

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