Knowing the disdain for Comcast gateways, I studied the specs for the TC8305C to determine compatibility issues. The manual says on page 4 "Here's what you get when you subscribe to both Xfinity voice and Internet services: Battery Back-up - Lithium Ion Battery back-up (up to 5 hours talk time and 8 hours standby time. Note: Battery back-up is only available if you subscribe to Xfinity Voice service."
This document is still current (6/14/'13) and makes no disclaimer suggesting that the consumer has to pay extra to obtain the back-up batteries.
My wife has severe medical issues, and Comcast clearly implies that this modem offers extended usage during a power outage, which we have often. In fact yesterday during an interruption is when I discovered the batteries were missing. Comcast has opened themselves to a huge liability, and I for one, would have nailed them had the phone failure resulted in injury.
Inquiring, I discoved that Comcast now charges exorbitant sums for these batteries ($40+ x2) and has apparently effectively eliminated any alternate sources. At least I have been unable to find any 3rd party sellers.
Some have suggested buying a UPS to support the device. Poor thinking for the following reasons:
1. It is inappropriate for a lessee to maintain rental equipment.
2. Why should I pay for a large piece of equipment to do the same thing the correct batteries should do and require no additional real-estate, in what is commonly a crowded environment.
3. UPS devices are more expensive than the batteries that belong in the gateway, and mean more batteries that eventually need to be recycled and eventually end up in landfills.
4. UPS devices use energy constantly. Redundant UPS's is not good energy management.
5. UPS devices require additional resources. (A place to plug in, with provisions to manage the added cordage, etc.)
6. The end user UPS's employed to support CPU's and related are designed for short term use to allow orderly shut-down of equipment to prevent date loss...using this set-up for the gateway might significantly reduce the service duration of the phone.
There are more reasons, but you get the idea.
The legal department for Comcast needs to be made aware of this vulnerability. The potential for loss could buy many, many batteries. It is a poor policy decision based on fattening the bottom line without concern for the well-being of it's clients. Many people reading this will not be pleased with Comcast, knowing that in case of a power loss, a reliable phone is being denied them solely based on maximizing profits.
I have basically the same problem as you. I told the rep that I would not pay for the battery and to send me a return UPS label. I was sent to 1-855-652-3446 and got an American service rep and they are sending me a free battery and a UPS label for the old stuff. Hope this helps. BE Firm!