My cable modem, which I rent from my ISP and through which I receive VOIP service, had to be replaced recently. The replacement modem did not come with a battery backup for the VOIP module, so I lose phone service when the power goes out. There's a battery available, but the ISP requires that I buy the battery for the modem that I rent.
When this modem's time comes, I'm keeping the battery. It might be a proprietary form factor and have proprietary connectors, so that it's not compatible with anything but this cable modem, but it's my battery.