The answer is that no Quality of Service (QoS) configured is the most likely cause of choppy VoIP calls. Without QoS, the network treats voice packets the same as data packets like file downloads or web browsing, so when the single flat subnet becomes congested, voice traffic suffers from latency, jitter, and packet loss—directly causing choppy audio and dropped calls. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding that VoIP is real-time and intolerant of network congestion, while data traffic is bursty and can wait. A common trap is blaming the phone hardware or bandwidth alone, but the core issue is the lack of traffic prioritization. Remember the memory tip: “Voice hates waiting—QoS gives it the fast lane.”
220-1201 Network Troubleshooting Practice Question
This 220-1201 practice question tests your understanding of network troubleshooting. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company deploys a new VoIP phone system. Users report that calls are choppy and sometimes drop. The network uses a single flat subnet with no QoS. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
There is no Quality of Service (QoS) configured.
VoIP traffic is sensitive to latency, jitter, and packet loss. Without QoS, data traffic can congest the network, causing choppy audio and dropped calls. Implementing QoS prioritizes voice traffic.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The VoIP phones have incorrect DNS settings.
Why it's wrong here
Incorrect—DNS issues would prevent registration, not cause choppy audio.
✗
The switch ports are set to half-duplex.
Why it's wrong here
Incorrect—half-duplex would cause collisions and poor performance, but the symptom is more consistent with congestion.
✓
There is no Quality of Service (QoS) configured.
Why this is correct
Correct—without QoS, voice packets compete with data traffic, leading to jitter and dropped calls.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Incorrect—Cat5 supports 100 Mbps, which is sufficient for VoIP; cable category is unlikely the cause.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 220-1201 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
Network Troubleshooting — This question tests Network Troubleshooting — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: There is no Quality of Service (QoS) configured. — VoIP traffic is sensitive to latency, jitter, and packet loss. Without QoS, data traffic can congest the network, causing choppy audio and dropped calls. Implementing QoS prioritizes voice traffic.
What should I do if I get this 220-1201 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 220-1201 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A customer complains that their VoIP phone intermittently drops calls and has poor audio quality. The phone is connected to a switch port that also carries data traffic for a nearby workstation. What is the most likely cause?
medium
A.The switch port is set to half-duplex.
B.The network cable is too long.
✓ C.Quality of Service (QoS) is not configured on the switch port.
D.The phone’s firmware is outdated.
Why C: VoIP is sensitive to latency and packet loss. When a switch port carries both voice and data without proper QoS, data traffic can congest the link, causing dropped packets and poor audio. QoS prioritizes voice traffic to prevent this.
Last reviewed: Jun 18, 2026
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