Xfinity Landline VOIP Phone Has Echo

My modem/router was recently upgraded by Xfinity to the xFi Gateway (the tall white modem with no lights on the front). I have a phone line through Xfinity Voice, and ever since the upgrade, when I place a phone call, I can hear my own voice echo back to me.

I have tried 3 separate phones to ensure it was not an issue with the phone itself. I then tried these same phones on an old fashioned copper landline, and they all worked correctly.

I have Xfinity out to check out the issue, and they swapped the phone jack that I had been using. They also ran new cable from the nearest junction box to my house, into my house, and all the way to the modem, so I do not think that is the issue. Despite all of that, the issue persists.

Edit: Want to add, the echo does NOT happen if I receive an incoming call. Only when I place an outgoing call.

Any help would be appreciated.