I notice that to get bt infinity you need to have a phoneline with bt as well? Is this a technical reason? Does your fibre branch off from the telephone line outside your property?
The technology being discussed here is FTTC - fibre to the cabinet. The fibre runs to the green cabinet which serves your locality, this cabinet houses a VDSL2 DSLAM which then serves broadband via your normal telephone line via VDSL2. No actual fibre comes into your home, you just have a DSL modem and DSL splitter the same as you had in the ADSL days

