Nighthawk C7100V modem/router for Xfinity

Something to have the ISP check the phone service lines up to the modem. 

Please post a copy and paste of the modems connection status and event log page.
https://kb.netgear.com/30007/How-do-I-obtain-the-cable-connection-information-from-a-NETGEAR-cable-modem-modem-router
https://kb.netgear.com/30008/How-do-I-view-or-clear-the-event-logs-on-my-NETGEAR-cable-modem-or-modem-router

Have the ISP check the signal and line quality UP to the modem. Be sure the ISP provisions the modem correctly.
Be sure there are no coax cable line splitters in the between the modem and ISP service box. 
Be sure your using good quality RG6 coax cable up to the modem. 
Start with removing any amplifiers, signal attenuators, or splitters from the coax.
From there check the line for kinks, damage, moisture in the line.
Check the connectors for improperly made ends, foil touching the copper coax line, loose connections, bad/old/cheap connectors, or corroded connections. Replace them if you do.
Be sure to power OFF the modem for 1 minute then back ON.
https://community.netgear.com/t5/Cable-Modems-Routers/General-info-and-Troubleshooting-for-Cable-Modems/m-p/1530376#M12853
https://kb.netgear.com/24311/Power-level-guidelines-for-a-NETGEAR-cable-modem-router
https://www.duckware.com/tech/solving-intermittent-cable-modem-issues.html

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foup wrote:

Bought this router because I have Xfinity phone line at home.  Everyday 2-3 days I lose the dial tone and have to unplug the router to restart them.  Internet and TV are not affected since they keep on working.  All connections are OK so what could be causing this?

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Could be a problem with how Xfinity has your account provisioned on their end.  The only way to check/resolve is through Xfinity support.

If internet/tv still work fine, its something xfinity needs to troubleshoot as only they have access/ management of the phone side.