VoIP Dropped Calls and Latency After Deployment: QoS or Firewall?
Short answer
D Misconfigured QoS is the most likely cause when VoIP calls drop and audio latency occurs while file transfer, web traffic, and speed tests show healthy data connectivity.
Question
• Dropped calls are occurring.
• Local file transfer is successful.
• New firewalls have been recently added.
• Web traffic is functional.
• A speed test returns expected results.
• Users experience audio latency.
Which of the following is the most likely cause of the issues?
Key Takeaways
This question tests whether you can separate VoIP quality-of-service symptoms from basic connectivity failures, and the trap is blaming the newly added firewall for a rule conflict.
VoIP quality problems such as dropped calls and audio latency after deployment point to misconfigured QoS when general data traffic and speed tests remain normal. The correct answer is D.
Common mistake: A conflicting firewall rule is tempting because new firewalls were added, but a firewall policy problem usually blocks or mangles specific signaling/RTP flows rather than leaving web, file transfer, and speed tests healthy while calls connect with latency and drop.
Expert Analysis
Why the voice path is suspect The evidence separates reachability from quality. Local file transfer works, web traffic works, and a speed test returns expected results, so the data path has addressing, routing, and adequate throughput. The failures are specific to VoIP: dropped calls and audio latency. That pattern points to real-time traffic not being prioritized, so misconfigured QoS (D) is the best fit. A comment summarizes the same logic: VoIP is extremely sensitive to latency, jitter, and packet loss, while healthy web, file, and speed-test outcomes suggest bandwidth is sufficient but voice is not being properly prioritized.
Adjacent decisions the question is really testing The newly added firewalls are the strongest distractor. A firewall-policy problem is adjacent to this scenario, but it normally produces a reachability failure: registration blocked, signaling denied, one-way audio because RTP is filtered, or calls failing only to certain destinations. It does not naturally explain calls that connect but suffer audio latency while every general data test is healthy. An incorrect voice VLAN (C) is also adjacent, because phones might land on the data VLAN or fail to get the correct voice scope; the usual symptom is phones not registering or no voice path, not clean data performance plus degraded audio. Spanning tree issues (B) would typically affect more than voice and could cause broadcast storms or intermittent loss across the segment.
What would flip the answer The answer would move toward A if the scenario said calls fail only when crossing a specific firewall interface, RTP is blocked, or signaling to the PBX is denied while other traffic is unaffected. It would move toward C if the phones had no voice VLAN, wrong DHCP scope, or could not reach the call manager. It would move toward B if there were topology loops, MAC flapping, or widespread intermittent outages. With latency and dropped calls on a working data network, D remains the most defensible.
Operational consequence If QoS is left misconfigured, voice packets compete with bulk data and can be delayed, jittered, or dropped during congestion, producing poor call quality and user distrust even though the network "passes" a speed test. Verifying DSCP markings, trust boundaries, and voice queue configuration is the practical follow-up.
Official Reference
These links open the official vendor documentation in a new tab.
Exam Strategy
On VoIP troubleshooting questions, first separate quality symptoms from reachability symptoms: latency, jitter, and dropped calls with normal web/file/speed-test results point to prioritization, while one-way audio or no registration points to firewall, NAT, or VLAN issues. Check whether a recent change touched QoS markings, trust boundaries, or queue configuration before blaming the change just because it is new. If data traffic is healthy, eliminate options that would break general connectivity.
Frequently Asked Questions
Why is a conflicting firewall rule less likely than misconfigured QoS when calls drop?
What VoIP symptoms specifically point to QoS rather than VLAN or spanning-tree problems?
What QoS settings should be verified after a VoIP deployment?
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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ERB14 👍 1 Answered DMisconfigured QoS: Voice over IP (VoIP) traffic is extremely sensitive to latency, jitter, and packet loss. Dropped calls and audio latency—while other standard network traffic (web browsing, speed tests, and file transfers) remains fully functional—indicate that network bandwidth is sufficient, but VoIP traffic is not being properly prioritized.
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