And YES, I did suspect this is a SERVICE issue, and not a "phone" issue.
Let me explain a bit:
When I hear the symptoms you mentioned - specifically that ONE SIDE of the conversation can not be heard - 80% of the time this type of issue was traced back to the phone service provider's equipment (vs. home equipment & wiring).
BUT, when a "digital" phone service is involved (like Comcast and most "cable companies"), that percentage goes UP - to about 95% of the time - or HIGHER.
So the MOST LIKELY CAUSE of this issue is the Comcast equipment or the "service" that is driving that equipment. Now specifically, you probably have a "large BOX" in your home that is a "gateway" for ALL of the services you get from Comcast (phone, TV, internet, etc.). And quite often that gateway needs to be "updated" with the latest version of control software, so that ALL of your services are up to date and working.
This update USUALLY happens automatically at various times, and you would not notice that it has happened, UNLESS something changes in one of your services, or if an update did not get applied properly. When that happens, one (or more) of the services can start to break down - like what is happening with your phone service.
So what I believe needs to be done is one of two things:
1) "Reset" your home "gateway" device yourself OR
2) Have the Comcast Technical support folks reset that gateway for you.
If you want to try to reset it yourself, it is really quite easy. All you need to do is LOCATE the gateway (the LARGE BOX with a bunch of lights on the front and several cables in the back), locate the POWER SUPPLY or POWER CABLE in the back of the box, and disconnect the power for a FULL TWO MINUTES. Then reconnect the power and WAIT another 2-3 minutes - for the gateway to "reload & reboot" ALL of the services and any software updates that may be needed.
I mention this because this is something that most folks can do, and more than 90% of the time this fixes such issues like you are having.
BUT if you are more comfortable contacting the Comcast Technical Support folks, then you can do that using the main contact number(s) I have below. NOTE: The Technical Support folks are there 24 hours a day, so they should be able to help you with this issue at any time of the day or night. Once you get to a support person, just tell them that you have the Comcast phone service and that you can NOT HEAR the person on the other end of the phone (or that they can no hear YOU) when calls are attempted.
NOTE: You MAY have to call them via a WORKING phone - such as a cell phone or neighbor's phone.
I hope this helps explain what I suspect is happening. Please let me know what you find or if you have any more questions.
Thanks, ***** *****