220-1201 Practice Question: A technician is troubleshooting a VoIP phone that…

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 (Session Initiation Protocol) is the crucial signaling protocol responsible for initiating, managing, and terminating VoIP calls. Its primary function in this context is to register the VoIP phone with the call server (often a SIP proxy or IP PBX), allowing the phone to announce its availability and receive incoming call information. If SIP traffic, typically on UDP/TCP ports 5060 or 5061, is blocked by a firewall, misconfigured on the phone, or the server address is incorrect, the phone cannot successfully register, preventing it from making or receiving any calls. This directly explains why the phone "cannot" function.

• ✗

  RTP

  Why it's wrong here

  RTP (Real-time Transport Protocol) is responsible for the actual transmission of audio and video data streams *during* an active VoIP call. It operates only after a call has been successfully established and set up by a signaling protocol like SIP. Since the problem indicates the phone "cannot" (implying a failure to even initiate or receive calls, which requires prior registration), RTP is not the cause because media streams are not yet being transmitted. Issues with RTP would manifest as one-way audio or no audio *during* an established call, not a complete inability to function.

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  DHCP

  Why it's wrong here

  DHCP (Dynamic Host Configuration Protocol) is a network protocol used to automatically assign IP addresses, subnet masks, default gateways, and DNS server information to devices on a network. The existing context implies the phone has an IP address and basic network connectivity, as it can likely communicate on the network. If DHCP were the root cause, the phone would typically lack an IP address entirely or have an incorrect one, preventing any network communication whatsoever, including the ability to ping other devices, which is not the scenario described.

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  DNS

  Why it's wrong here

  DNS (Domain Name System) translates human-readable hostnames, such as "voipserver.example.com," into numerical IP addresses that computers use for communication. While a VoIP phone might use DNS to resolve the hostname of its SIP server during the registration process, the problem implies the phone has basic network connectivity and can potentially reach the server by its IP address. If the phone can successfully ping the call server's IP address directly, it confirms that network routing is functional and the server is reachable, indicating that DNS resolution failure is not the fundamental cause of the phone's inability to function.

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