No, you're not alone!
Both my wife and I have experienced this on a nearly continuous basis since we got the HTC One. The volume goes up and down a few times after a call is made and then usually stays so low you can't hear the other person. This seems to be a problem only with the combination of the HTC One and the this particular wireless extender. Other people have used the Wireless extender when they're over and had no problems... unless they own an HTC One.
Strangely enough, even working off the Verizon towers, you can still hear a slight yo-yo-ing of the sound level during a call... it is very subtle. It is almost as if there is some sort of volume auto-compensation code in place.
Tried working with Verizon and HTC, HTC is extremely rude and claims their phones are "as is" and don't guarantee that they will work with anything else. Verizon has been a bit more helpful and tried a number of things since August including replacing the phones, but really the only thing that fixes it is to pull the plug on the wireless extender and work off the towers... although they are weak in our area.
A couple of patches ago, it magically started working, so I assumed it was a permanent fix. Unfortunately, this week the problem has returned and with a vengeance; it is worse than ever. The volume now drops to such a low level that even with the speaker phone turned on you can barely hear the caller. As a bonus, now the phone calls are contaminated with a large amount of static to boot. The extender still works great with my friend's phones but I've had to just pull the plug on it and lean up against an outside wall to get my calls to go through.
Eight months of this non-sense is about enough. I can't believe they still have found a solution.