From your description, it appears that building developer has only done part of the job -- or has not bothered to read and/or understand the
Openreach document titled
"How to Build a Fibre Network. Developer Guide." My copy is Version 4 - March 2017.
Without having sight of what you can see, I suspect that your assumption is correct. The way I would proceed is as follows --
- Go to each extension socket, in term, and make a note of how they are connected.
- At the back-box, under the stairs by the ONT, connect a tone-tracing oscillator and check for the signal at each extension socket.
- Consider the results of the above two steps.
- Connect a standard extension socket to the cable and mount it onto the under-stairs back-box.
- Obtain a type BS6312 431A to BS6312 431A patch cable. (That is just a cable with standard telephone plugs at each end.)
- Plug one end of the above patch cable into the telephone port of the ONT and the other end into the newly fitted extension socket.
- Confirm that telephone calls can be made and received.
As you will appreciate, there is a lot of "fine detail" that I have omitted in the above. Treat it as a preliminary guide.