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A Beginner’s Guide to Troubleshooting VoIP Phones


A VoIP phone off the hook
Image: magneticmcc -
While VoIP phones offer far more features and better clarity than legacy phones, troubleshooting VoIP phones requires voice and networking skills. It isn’t necessary to be CCNP certified. However, understanding the fundamentals of networking is very helpful. I’d recommend Cisco Certified Entry Networking Technician (CCENT) or Microsoft Networking Fundamentals (MTA) for anyone entering telecommunications.
When it comes to VoIP phones, just because you hear a dial tone in the handset, it doesn't mean you have a PBX (cloud or on-prem) connection. The dial tone sound is just an MP3 recording. The phone will play a dial tone when it is off the hook, and it has registered to the service provider, even if it is not fully functional. This can occur if some ports on the firewall are open (such as HTTP/HTTPS/FTP), while other ports for talk path (UDP) might be blocked. The dial tone sound is good to hear when troubleshooting and a step in the right direction, but it doesn’t necessarily mean that the phone is working properly.
Knowing some basic network information ahead of time can be a valuable aid when troubleshooting. I recommend the following information: the network address, subnet mask, VLAN ID, and DHCP scope for both the voice and data. Having this information documented during installation projects and keeping it current as part of normal operations will help you to make troubleshooting manageable.
The first step in troubleshooting is to understand the problem. Your company’s help desk ticket may lack details because users typically just say things like, “My phone isn’t working” or “My phone won’t ring” or “I can’t make a call.” These descriptions vary as much as the people we support. Some may claim a phone doesn’t work simply because a vendor was unable to reach them once, and the problem was on their end.
Unless you are troubleshooting a common-area phone, check that the phone is in the user’s name. It’s not uncommon for modern phone systems to utilize active directory integration. Check the profile of the phone and make sure that it’s assigned to an active employee with a working network ID. I’ve seen tickets in which the new hire just assumed ownership of the previous employee’s phone that had expired credentials.
Whenever possible, you should get out of your chair and walk to the person who’s having the problem. It means a lot to people to meet them face to face. It builds rapport, shows you care and gives you a chance to see first-hand what’s going on. There’re exceptions to this rule, like anything else in life, but if you can, visiting the person face-to-face pays dividends in the long run.
Is the phone plugged in, or does it get power over the network? Did it get unplugged from the wall and plugged back into the wrong ethernet wall jack? Check the network cables in the back of the phone. Sometimes, users will relocate offices or do deep cleaning and reverse the network and PC ports when plugging everything back in. Is the phone screen illuminated? Swap out the power supply if necessary or see if power over Ethernet (POE) was disabled on the switch. Has the network switch failed? That may not be obvious if the user’s computer is on Wi-Fi. If the cabling looks proper, a reboot is in order. Phones are miniature computers, and often, a reboot will resolve the issue. However, if it’s a recurring issue, there may be a deeper issue.
If it’s a feature or behavior problem, it’s helpful to duplicate the problem. Make a call from their phone or receive a call from their phone. You may find that it’s something as simple as Do not disturb (DND) was activated. Show them what DND is and then ask how they or their kids are doing. Every interaction is an opportunity.
On occasion, audio quality can be an issue, but it can be resolved. Some common descriptions are:
  • Cracking, crackling, popping
  • Breaking up
  • Skipping or can’t hear all the words
  • Static
  • Jitter
  • One-way audio
The cause of poor audio could be the telephone, the network connection, cabling, switch, or even, a power sag caused by a portable space heater. Ask questions and examine the situation. Does it happen with all calls or just with the last caller with whom the user spoke? Does it happen with others in the office or only this user? Did the user change out the curly cord or curly-cord swivel adapter? Sometimes, those become loose or corroded with time. You may choose to swap out parts or the entire phone. Check the physical things first — the things we can touch. Always start with the simple and easy- moving toward the complex after each step.
Does the phone have an IP address? If the phone still doesn’t receive an IP address after a reboot, it’s time to check the port. Verify that the port is patched into the network switch and is active. It may be necessary to take a working phone and plug it into this port.
Another option is to plug a computer into that port to see if it’s working. Be sure to disable Wi-Fi on the PC during this step. Keep in mind though, that computers and phones should be on separate Virtual networks (VLANS). So just because a computer receives an IP address and functions doesn’t necessarily mean that a phone will. But it will aid in troubleshooting to at least see if the port is active and the switch is operational.
VLANs are designed to segment traffic on the network. VLANs help to reduce jitter, latency, and loss when network traffic is heavy. Analogous to lanes in a highway, the carpool lane is designed to carry voice without delay while the rest of the cars (such as email) may slow down a bit during rush hour. The voice VLAN should be free and clear to handle phone calls even when email is delayed by a second or two.
It’s important to understand the methodology used in your environment to set up the phones. There are several options, including:
  1. The phones could be manually configured with IP addresses and URL registrations.
  2. The phones could be dynamically configured using DHCP and DHCP options.
  3. Phones may also be preconfigured to the carrier with the registration information.
In high-security areas, the network engineer may lock network ports to specific MAC addresses. In these cases, moves, adds, and changes require coordination with a network administrator or engineer. If your environment has grown and you find that a phone can’t pick up an IP address, the problem may be that the voice VLAN has consumed all the available IP addresses. Lease periodicity should be in line with your company’s best practices.
Some people have had bad VoIP experiences and hold misconceptions as a result. One is that VoIP provides poor audio quality. In fact, VoIP should have perfect audio. It should sound as if you are in the room with the person. The experience should be equal to or better than a traditional landline. If this isn’t the case, I would encourage you as a voice administrator to partner with your network engineer and do a deep dive into the factors that can shape the network traffic performance. As you can see, the relationship between voice and the network is like a house and its foundation. A strong foundation (the network) is required to support a house (voice traffic).
Another misconception is that it is a bandwidth hog and will slow the network down. In fact, VoIP is very lightweight. A clear conversation can be made with as little as 35-50k, though it varies a little depending on the carrier and CODEC employed. In addition, VoIP phones should be placed on a separate VLAN — keeping voice in its own carpool lane.
When troubleshooting VoIP phones, the best practice in a large company is to address the things you can control. If you still can’t find the issue, then engage a network administrator, network engineer, or security/firewall administrator and partner with that person to continue troubleshooting. Take the opportunity to learn and grow from the experience.

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