You need a new telephone socket incorporating a BT open reach socket. The top socket is for the modem and the bottom one for your telephone.
This will need an Engineer to install the new double socket. In my case the original router from TALKTALK incorporated the modem. With PlusNet you require a BT open reach modem PLUS a PlusNet Router, two separate items, both requiring a 13A socket.
In my case the Engineer never showed up and it took 3 telephone calls to get my modem and router posted to me. PlusNets customer service is appalling. When you phone them you have to hang on, in my case for up to 45 minutes before getting connected.Most unsatisfactory.
I hope you get your problem sorted soon.