For at least the past 6 months we have been having issues with our 3rd party VOIP system losing words while talking as well as problems connecting to websites. There have been times it is so bad with the phones that I will call Comcast and they will call me back on a cell phone because they can not understand what I am saying.
Over the past months I have used Wireshark many times looking at packet information. I have tried it plugged directly into the Comcast modem as well as our router and the data capture looks pretty much the same. I see a large number of retransmissions and duplicate Acks. 90+% of these are on the WAN side (when I have the computer connected to our network via the router) I was noticing a ICMP TTL error all the time in the packet captures. I finally got a technician out when a problem was occuring and he found a signal issue. Someone came and fixed that problem outside our building but the problems continued. Another tech came out and replaced our SMC modem with a Netgear one. The ICMP TTL problems changed to ICMP Port unreachable messages.
Again, it doesn't matter if I am using our internal network with the Windows Server 2008 R2 or connected directly to the modem and WAN. It doesn't matter if I am using the 10.1.10./24 our modem is set to or if I assign the computer one of our Static IP addresses.
The quality of our Internet is drastically affecting our phone calls and ability to work. This is for a dental office so issues with our phones directly affects our ability to get patients in and new patients won't always call back when it goes directly to voicemail. The fact that I have the same problems connected to the modem or our network really makes it apparent that this is a Comcast issue. I know I've started to see an IPv6 address at the modem so I sometimes wonder if this is a routing issues that is somehow tied to their implementing IPv6 in our area. We have a 5 block of IPv4 addresses.
I have discounted the router while testing directly connected to the modem, doesn't make any difference. I can't do any QOS with the phones plugged directly into the modem as the modem doesn't have that feature that I have seen. Witht he large amount of data on our network it seemed best to have them separate from it, hence being connected to the modem directly. Also, if there was a problem with the router (which has been replaced once already by Netgear to make sure it wasn't causing the packet drop issue on it's WAN port) then the phones would perform better being on the other side of it.
Once I called the local Comcast tech when I was having a problem and he couldn't even ping our modem from his location. He and another person came out and checked over everything outside the building and couldn't find any issues, but then it started to respond to him mysteriously. I've called him several times (he asked me to when we were having an issue) and the last 2 times I didn't even get a call back from him. He did bring up that someone above him thought it could be a routing issue and told me I would be getting a call from someone in that area, but I never have and that around Nov 7th so it has been a month.
It shouldn't be a bandwidth issue. I have tested this on the weekend, when the VOIP goes to voicemail automatically so it isn't using much of our bandwidth and having a single computer trying to surf websites I will get errors. I can be in nslookup at the command prompt and it will sometimes give 6-10 timeout messages before it finally gives me an IP address. The one computer with a browser and command prompt up, no updates downloading, nothing uploadi ng, isn't going to overtax our connection to Comcast.
I can even have that one computer connected to the Internet and go to speedtest.net and run multiple tests. My download speeds within 10 minutes can vary from 1MBs to 36+MBs.
here is my suggestion. Order a single Comcast phone number. They have to deliver it VoIP, same as your 3rd party provider. When things go to pot, try making a call on the Comcast VoIP. If there's a network or cable problem then your voice quality will be terrible on the Comcast voice and you can call and beat them over the head about it. Keep calling them and eventually they will fix whatever is causing their VoIP to screw up. Then you can just cancel their phone service.
I am seeing similar symptoms. Same 5 static IP configuration. Comcast swapped out the older SMC with a newer unit, but still having problems. I can disconnect the VOIP server and still have issues. Simultaneously pinging several of the IP addresses, including the gateway's IP from the outside, shows that a simple FTP of a large file from one of the servers, causing some traffic that lasts a while, will hammer the ping times simultaneously all of the IP ports. It seems like the Comcast gateway gets locked into handling that one request and ignores servicing the other IP addresses. Interestingly, instead of pinging from the outside, pinging from one of the 5 IPs to another always works perfectly.