- A
SIP
SIP is used to register VoIP phones with the call server; if SIP is blocked or misconfigured, registration fails.
- B
RTP
Why wrong: RTP carries audio streams after a call is established; it is not used for registration.
- C
DHCP
Why wrong: DHCP provided the IP address, which is working; the phone has connectivity.
- D
DNS
Why wrong: DNS may have been used to resolve the call server's name, but the phone can ping by IP, so DNS is not the issue.
Question 749 of 1,020
SIP Registration Failure: VoIP Phone Cannot Register with Call Server
This 220-1201 practice question tests your understanding of network protocols. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 technician is troubleshooting a VoIP phone that cannot register with the call server. The phone has a valid IP address and can ping the call server by IP. Which protocol is most likely not functioning correctly?
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.
Quick Answer
The answer is SIP, the Session Initiation Protocol, because a successful ping confirms that network-layer connectivity and IP addressing are working, so the failure must be at the application layer where SIP handles VoIP phone registration and call setup. Even when a device can reach the call server by IP, the registration process depends on SIP messages like REGISTER and 200 OK; if SIP is misconfigured, blocked, or not running, the phone will never authenticate or register. On the CompTIA A+ Core 1 220-1201 exam, this question tests your ability to isolate protocol-specific failures—a common trap is assuming a successful ping means everything is fine, but SIP operates above ICMP. Remember the memory tip: “Ping works, but no ring? Check SIP for the thing.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
SIP
The phone has a valid IP address and can ping the call server by IP, which confirms that Layer 3 connectivity and DHCP are working. However, the phone cannot register with the call server, which requires the Session Initiation Protocol (SIP) to exchange registration messages (REGISTER request and 200 OK response). If SIP is misconfigured, blocked by a firewall, or the SIP proxy is unreachable, registration will fail even though basic IP connectivity exists.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
SIP
Why this is correct
SIP is used to register VoIP phones with the call server; if SIP is blocked or misconfigured, registration fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
RTP
Why it's wrong here
RTP carries audio streams after a call is established; it is not used for registration.
- ✗
DHCP
Why it's wrong here
DHCP provided the IP address, which is working; the phone has connectivity.
- ✗
DNS
Why it's wrong here
DNS may have been used to resolve the call server's name, but the phone can ping by IP, so DNS is not the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common pitfall on the CompTIA A+ exam is confusing Layer 3 reachability (ping) with application-layer registration (SIP). Candidates may incorrectly suspect DHCP or DNS when the real issue is a misconfigured or blocked SIP protocol.
Detailed technical explanation
How to think about this question
SIP registration uses UDP or TCP port 5060 (or 5061 for TLS) and involves a REGISTER request sent to the call server's SIP proxy. The server responds with a 200 OK if authentication succeeds, or a 401/407 if credentials are required. A common subtle issue is that the phone may have the wrong SIP proxy address or port, or a firewall may be blocking SIP traffic while allowing ICMP (ping) and other protocols.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 220-1201 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Network Protocols — study guide chapter
Learn the concepts, then practise the questions
- →
Network Protocols practice questions
Targeted practice on this topic area only
- →
All 220-1201 questions
1,020 questions across all exam domains
- →
CompTIA A+ Core 1 220-1201 study guide
Full concept coverage aligned to exam objectives
- →
220-1201 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 220-1201 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Mobile Device Hardware Servicing practice questions
Practise 220-1201 questions linked to Mobile Device Hardware Servicing.
Mobile Device Connection Methods practice questions
Practise 220-1201 questions linked to Mobile Device Connection Methods.
Mobile Device Accessories practice questions
Practise 220-1201 questions linked to Mobile Device Accessories.
Mobile Device Network Connectivity practice questions
Practise 220-1201 questions linked to Mobile Device Network Connectivity.
Mobile Device Application Support practice questions
Practise 220-1201 questions linked to Mobile Device Application Support.
Network Protocols practice questions
Practise 220-1201 questions linked to Network Protocols.
TCP & UDP Ports practice questions
Practise 220-1201 questions linked to TCP & UDP Ports.
Wireless Networking Technologies practice questions
Practise 220-1201 questions linked to Wireless Networking Technologies.
Network Services practice questions
Practise 220-1201 questions linked to Network Services.
Network Configuration Concepts practice questions
Practise 220-1201 questions linked to Network Configuration Concepts.
Common Networking Hardware practice questions
Practise 220-1201 questions linked to Common Networking Hardware.
IP Addressing practice questions
Practise 220-1201 questions linked to IP Addressing.
Practice this exam
Start a free 220-1201 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this 220-1201 question test?
Network Protocols — This question tests Network Protocols — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: SIP — The phone has a valid IP address and can ping the call server by IP, which confirms that Layer 3 connectivity and DHCP are working. However, the phone cannot register with the call server, which requires the Session Initiation Protocol (SIP) to exchange registration messages (REGISTER request and 200 OK response). If SIP is misconfigured, blocked by a firewall, or the SIP proxy is unreachable, registration will fail even though basic IP connectivity exists.
What should I do if I get this 220-1201 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Keep practising
More 220-1201 practice questions
- During a network cable installation, a technician needs to verify that a newly run Cat6 cable is properly terminated and…
- A user connects a 4K monitor to their laptop using a USB-C port. The monitor is detected, but the resolution is stuck at…
- A technician is troubleshooting a laptop that will not charge. The battery is removable, and the power adapter works on…
- A customer brings in a smartphone with a broken charging port. They want the port replaced. During disassembly, the tech…
- A user reports that their laptop's keyboard types random characters when certain keys are pressed. The laptop has not be…
- A technician is troubleshooting a laptop that shuts down randomly after a few minutes of use. The fan is spinning, and t…
Last reviewed: Jul 4, 2026
This 220-1201 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 220-1201 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.