VOIP phones - static, dropped calls.

My client is using 8x8 Voip phones, which I have used for a number of years without a problem. They have Comcast internet service (not sure of the speed), a Meraki MX60 firewall. Their calls frequently have static and often get dropped entirely. Comcast has replaced their modem; 8x8 has done ping testing which shows normal one day, then latencies in the 400ms range the next. Computers seem to function fine. Questions:
1. What tests can I run to identify where the problem lies?
2. If you've seen this before, how did you resolve it?
3. Should Comcast be able to identify the latency source?

Any other ideas?
Thanks!
LTGAsked:
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Darrell PorterEnterprise Business Process ArchitectCommented:
A couple of things could be causing this:

What is the MTU set to on the Meraki's Internet interface and ethernet 2 (or wherever the voice switch is connected)?
For Comcast, this value is normally 1500.
To verify this, you need to be able to send a non-fragmented packed to a destination on the internet.  Fortunately, the Windows Ping tool can do this for us.
If the MTU needs to be 1500, we can use this command from a command prompt:
PING -f -l 1472 www.google.com
The -f tells Ping to flag the packet as "Do not fragment".
The -l (lowercase L) 1472 tells PIng to use a payload of 1472 bytes (1472 bytes of data plus the 28 bytes of packet overhead = 1500 bytes, our MTU).
If the ping is successful, then the MTU is set correctly.
If Ping returns a message indicating Packet needs to be fragmented but DF set. then the MTU on the outside interface of the Meraki needs to be set lower.

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LTGAuthor Commented:
Thanks, Walkabout. That's a good tool to keep handy. Unfortunately (or fortunately, I guess) the firewall passed the test, so that's not the culprit. But good thought! Thanks!
Darrell PorterEnterprise Business Process ArchitectCommented:
How is the Meraki configured for Quality/Class of Service?
Meraki calls this Traffic Shaping and a thorough description of this can be found
here.
Did you set the bandwidth correctly on the Meraki for upstream/downstream speeds?  Normally Comcast is asymmetric, meaning they provide 20-150 Mbps down and 2-20 Mbps up and the Meraki *MUST* be correctly configured to match as precisely as possible the actual throughput you experience.
Also, please review these considerations on 8x8's website.  8x8 may also have some Comcast-specific settings they can employ on their end to improve the customer's experience.
LTGAuthor Commented:
I am being invited back to test these possible solutions soon, but wanted to close this since its been here a while. Thanks for your suggestions. I will try to get back and let you know what the final resolution was.
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