Ah, I was looking at Kitz’ pictures on the main site, didn’t realise that thinks are now different -?
The BBU only powered the ONT, nothing else. You still need a router to access the internet.
It's only purpose was to power the ONT so customer could make voice calls via the ONT's voice port.
Openreach ONT's no longer have voice ports. They no longer sell the product that used the ONT voice port (FVA - fibre voice access).
Openreach want out of the voice business. They are retiring PSTN and WLR and voice is moving to the ISP's responsibility. That's why they are all launching VOIP products.
The newer ONT's are as small as a master socket. They come with no enclosure and no BBU.
Openreach haven't supplied BBU's for a few years now, though you can still purchase them on the likes of eBay.
Powering the ONT alone is useless though, just buy a UPS.