Home phone line: Caller cannot hear me later in call

Over the last several weeks while on a call using my Xfinity home phone voice line, my voice transmission stops. I can hear the person on the other end, but they cannot hear me. This can happen within a few minutes, or 5 minutes or a bit longer, but the call always drops my voice. I haven't been able to have a complete call for weeks and have to revert to my cell which isn't the best signal where I live. I fear someday a call to emergency services using my Xfinity phone could be disastrous. 

I have done many hard power-cycle resets on the Gateway with no change. Today I spoke with a Xfinity agent and have the above logged. I told the agent I would do some "self troubleshooting" today. See below. Note that I have NOT had Internet access issues at all. The problem is solely with the home phone line.

Also, earlier this week Xfinity installed a completely new Coax cable from the street pole to the inside of the house where the Gateway resides. This was due to noise they saw on the line in their testing the week before. I told them about the above (loss of my voice during a call)issue before they changed the cable and that the new cable "might" correct it. It has not. The problem still exists.

Debug Info/"Self Troubleshooting"

According to the Gateway http://10.0.0.1 direct access.....

Hardware:

My Portable Phone Base is now on the TEL 1 port. For the year prior it was on a extension wire to another room. (Edit: This recent problem never occurred during the time the phone was on the extension jack)

So I put a wireline phone on TEL 1 port and the same problem happened today on several "test" calls to or from my cell.

I've seen this problem discussed in several threads here in the past, with no specific finding posted. Here is a example:

https://forums.xfinity.com/conversations/home-phone-service-equipment/caller-cannot-hear-me/6101c08a6e891e630f65e008

Before I contact Xfinity again for guidance, is there other testing I can try to help diagnose this annoying intermittent fault?

Rich