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220-1201 Practice Question: That their VoIP phone works but the connected PC…
A user reports that their VoIP phone works but the connected PC cannot access the network. The phone is connected to the wall jack, and the PC is connected to the phone's passthrough port. What is the most likely cause?
⚠ Common exam trap
The 220-1201 exam often tests the misconception that a working phone implies the entire port is correctly configured, when in fact the phone can operate on its own voice VLAN even if the data VLAN is missing or misconfigured, leaving the PC stranded.
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
✓
The switch port is not configured for the data VLAN, so the PC cannot get an IP address.
The VoIP phone and PC share a single physical switch port via the phone's passthrough port. For the PC to access the network, the switch port must be configured as a trunk (or access port with voice VLAN) carrying both the voice VLAN (for the phone) and the data VLAN (for the PC). If the switch port is not configured for the data VLAN, the PC will not receive an IP address from the DHCP server and cannot communicate on the network, even though the phone works because it uses its own voice VLAN.
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 PC's network cable is faulty.
Why it's wrong here
If the PC's network cable, or the segment connecting the phone to the PC, were faulty, the PC would experience a complete loss of connectivity. However, the VoIP phone itself is reported as working, which indicates the primary network connection to the wall jack and the phone's internal networking components are functional. While a specific cable segment *to the PC* could be bad, the phone's operational status points away from a general cable fault as the primary cause for the PC's lack of connectivity.
- ✗
The VoIP phone's passthrough port is disabled.
Why it's wrong here
Many enterprise VoIP phones include an integrated switch, providing a passthrough port (often labeled PC) to connect a computer using a single wall jack. While disabling this port would prevent the PC from obtaining network access, most phones have this feature enabled by default for convenience. It would require a deliberate administrative action, either locally on the phone or via a centralized management system, to disable the passthrough functionality, making it an uncommon default issue.
- ✓
The switch port is not configured for the data VLAN, so the PC cannot get an IP address.
Why this is correct
This is the most probable cause. VoIP phones commonly operate on a dedicated Voice VLAN, while PCs require access to a separate Data VLAN. If the switch port connecting the VoIP phone is configured only for the Voice VLAN (e.g., as an access port for the voice VLAN or with an incorrect native VLAN), the PC's data traffic, attempting to join the Data VLAN, would be dropped. Consequently, the PC would fail to obtain an IP address via DHCP, even though the phone successfully communicates on its designated VLAN.
- ✗
The PC's network adapter is set to a static IP address that conflicts with the phone.
Why it's wrong here
An IP address conflict occurs when two or more devices on the same network segment are assigned identical IP addresses. If the PC's network adapter had a static IP address that conflicted with the VoIP phone, both devices would experience intermittent or complete network connectivity issues, not just the PC. Since the VoIP phone is reported as working correctly, an IP address conflict between the PC and the phone is highly improbable.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.