We have used Comcast VOIP, with the same Panasonic phones for years. Today, when we call out, the other party answers but cannot hear us. We hear them. This happens regardless of whether we use a hard-wired phone, the Panasonic base station or one of the satellite phones.
By chance do you have any advanced call forwarding enabled on your account (The mobile app and/or nomorobo for example)? How about the other person (if they are using Xfinity)?
I also have Panasonic phones and am experiencing similar issues.
No, I let the base phone do all of my advanced features. For years, the xfinity phone worked fine. This last week, it began have sporadic fits. Everything works fine, then suddenly the phone goes dead. One time, I could not even get a dialtone. Ten minutes later, everything works fine again. An hour later, I can hear the other party but they cannot hear me.