This article aims to collect the basic information necessary to identify a problem with Voice over IP and to satisfactorily investigate this problem by making an analysis adapted to each scenario.
When troubleshooting VoIP issues, we recommend noting the answers to the following questions:
Who is the phone vendor? Is the phone hardware or software based? Note the manufacturer, model number and relevant versions.
Who is the PBX vendor?
Which VoIP protocols are in use (H.323, SIP)? Is the vendor completely H.323 or SIP Compliant?
Which Audio CODECS are being used? On the LAN and over WAN or VPN?
Does the issue impact internal calls only, external or both? Where is the caller? Where is the person being called? Note the answers in relation to the SonicWall appliance in your network.
Do calls get established?
Is there audio (one way / two way)?
Location (in relation to the SonicWall appliance) of the Gatekeeper, Call Manager or Proxy? The answers to these questions determine how the calls are set up and if the SonicWall should be part of the call setup.
What is the behavior of the phone and/or call?
Are phones able to register?
Can users of the phones hear a dial tone?
Does the call connect?
Can both sides hear the conversation?
What does the phone quality sound like?
Does the call drop over time?
What type of WAN connection are the SonicWall(s) using and how is it set up to meet the needs of the VoIP implementation?
What is the latency and total available bandwidth of the pipe?
What is the average or typical number of concurrent calls?
How much bandwidth does the VoIP implementation require per call?
Is there enough total bandwidth for the traffic, including VoIP and everything else?
Has fragmented packet handling been enabled on the SonicWall?
What is the current WAN interface MTU value?
Is Bandwidth Management (BWM) and/or Quality of Service (QoS) enabled?
Is BWM correctly configured?
Is the type of QoS correctly configured?
Are H.323 and/or SIP transformations enabled in the SonicWall? Did you reboot the SonicWall after changing these values?
Gather or generate the following information for use as an important part of answering the above questions and successfully troubleshooting the VoIP issue:
Packet capture. This is the most important tool. Wireshark allows the actual playback of calls and an in depth look at the call flow.
Technical Support Report(s) (TSR).
Exported preferences file(s) (EXP).
Packet captures and traces.
Detailed network diagram showing all components and IP addresses.
If you should require technical support in order to resolve a VoIP issue, we ask that you provide as many answers to the above questions and as much of the requested information as possible so our technical support agents may help you reach a prompt resolution.