Comcast no longer supplies batteries. You can buy one by calling 1-888-972-1261.
Nerdburg, when did this happen? Because I am pretty sure they do still offer them at 35.00 for their rental equipment.
@Nerdburg was saying that Comcast doesn't supply them free, that they have to be bought. The number he supplied is where you call to order one. Depending on the model of the voice modem batteries range from $35-$70 plus tax and shipping.