Business Phone Systems Troubleshooting & Maintenance 2 — Questions and Answers
Question 1: What should you check if phones on a specific subnet are experiencing call quality issues while others are not?
- QoS configuration on the network switch or router serving that subnet (Correct answer)
- Whether those phones have an active voicemail plan
- The firmware version on the affected phones only
- Whether the SIP trunk has enough channels
Correct answer: QoS configuration on the network switch or router serving that subnet
Localized quality problems often indicate that QoS markings are not being applied or honored on the switch or router for that specific network segment.
Question 2: What is a common cause of calls dropping after exactly 30 or 32 seconds on a VoIP system?
- A firewall with a short SIP ALG or UDP session timeout killing the RTP stream (Correct answer)
- Maximum call duration limit set to 30 seconds
- Insufficient codec bandwidth for the call
- A licensing limit on the number of call minutes
Correct answer: A firewall with a short SIP ALG or UDP session timeout killing the RTP stream
Many firewalls and SIP ALGs time out short-duration UDP sessions, cutting off RTP media streams mid-call at predictable intervals.
Question 3: What does SIP ALG (Application Layer Gateway) do and why is it often disabled for VoIP?
- Modifies SIP packets passing through a router, often corrupting them and causing call failures (Correct answer)
- Accelerates SIP call setup by caching registration data
- Encrypts SIP traffic between the router and provider
- Prioritizes SIP packets over other traffic types
Correct answer: Modifies SIP packets passing through a router, often corrupting them and causing call failures
SIP ALG attempts to translate SIP headers for NAT but frequently corrupts the data, causing registration failures, one-way audio, and dropped calls.
Question 4: What is the recommended approach when a firmware update breaks IP phone functionality?
- Roll back to the previous stable firmware version from the manufacturer's portal (Correct answer)
- Replace the phone with a newer model
- Factory reset the phone and re-provision it with the new firmware
- Disable auto-update and leave the broken firmware in place
Correct answer: Roll back to the previous stable firmware version from the manufacturer's portal
Rolling back to a known-good firmware version is the fastest way to restore functionality while the manufacturer investigates the problematic update.
Question 5: What information should be collected before contacting VoIP provider support about a call quality issue?
- Affected extension numbers, time and date of issue, call recordings, and network test results (Correct answer)
- Only the account number and billing contact
- The model of every phone in the office and their purchase dates
- The names of all employees who reported the problem
Correct answer: Affected extension numbers, time and date of issue, call recordings, and network test results
Specific call details, timestamps, recordings, and network test data allow provider support to reproduce and diagnose the issue efficiently.
Question 6: What does a '503 Service Unavailable' SIP error response indicate?
- The SIP server is overloaded or temporarily unavailable (Correct answer)
- The destination extension does not exist
- The caller is not authorized to make outbound calls
- The called number is busy on another call
Correct answer: The SIP server is overloaded or temporarily unavailable
A SIP 503 response means the server cannot handle the request due to overload or maintenance, similar to an HTTP 503 error.
Business Phone Systems Troubleshooting & Maintenance 2
What should you check if phones on a specific subnet are experiencing call quality issues while others are not?