Actually I believe the battery is the phone companies responsibility, because it is their responsibility to ensure phone service remains during a power outage, just as it is with POTS. The fact that the power is now supplied via CPE rather than down the copper line from CO does not change that.
They know however that most people don't know and will just take their word for it and buy the replacement battery. Long as nobody says anything about it and pushes back they will get away with it. And it is money in their pocket. And once it is done long enough everyone will forget about it being the phone companies responsibility and then they have successfully created a defacto standard.
If they refuse to replace it. Complain to your PUC and your State Attorney General.
They just continue to erode away what customers perceive to be their responsibly. Until they are no longer responsible for anything except collecting the money. That is the game they are playing with customers.
It may take a tragedy and a lawsuit for them to get the message though.--
Be a Good Netizen - Read, Know & Complain About Overly Restrictive Tyrannical ISP ToS & AUP »comcast.net/terms/ »verizon.net/policies/
Say Thanks with a Tool Points Donation