I've noticed the same issue with the XB7. The below article might explain it: https://nationalinterest.org/blog/buzz/why-comcast%E2%80%99s-new-wi-fi-6-router-fails-i-spent-three-days-testing-it-121796. Hopefully, as the article suggests, it's a firmware issue that will be resolved. I can't, however, recommend another cable modem with voice. I tried the Arris TM25 and the Netgear CM1150v modems. They would work for a time, slow down, lose connection, work for a time, slow down, then lose connection. Those modems are both supposed to work with Xfinity's gigabit tier, and they probably do in most locations, not mine. I had techs come and go 4 or 5 times with giving me stable internet until they installed the XB7. It may be inconsistent, but it stays connected. Unless you were able to get another cable modem to work for you, keep fighting to get full gigabit speed. If you have fixed the problem, or fix it in the future, let me know. I fight for full speed after I have recovered from my eventually successful efforts to finally get stable internet.
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