@oilcloth a further clarification. The "adapter" you need is a network device (usually called an ATA - Analogue Telephone Adapter) that connects to your router via a standard ethernet cable, so you need a spare ethernet port on the router for it.
Plusnet have nothing to do with such VOIP phone services, you need to obtain the necessary kit and set up a VOIP account yourself. Some companies can supply both the service and the equipment.
IMPORTANT if you think your router has a phone socket (covered up usually) - NO IT DOESN'T. That socket does not work with the PlusNet router, though the very similar BT routers have a similar socket and can use it ON BT BROADBAND services, which causes regular speculation and confusion.
