The Virgin (Ambit?) 255 or 256 models, which can cope with the changes they are making to the network that older models may not work well with (if at all).
IIRC it's to do with them changing from one (very old) standard to a newer more efficient one, and also trying to standardise the user equipment as at the moment they have something like 12 different models of modem in use, some of which haven't been supplied by them for about 10 years.