My original master socket was a recessed one, and the house was built in the early seventies.
Ah but while house builds of that decade [70s] might well of had flush phone sockets, the concept of a master socket being the demarcation point between the incoming service and extension wiring had yet to be introduced. For example, an exterior, or interior, junction box for a single line often had wiring spun off in all different directions to several flush sockets throughout the home.
In my case the internal phone wiring was installed, with flush sockets, by the builder with an array of un-terminated cables ending up in the loft. When ordering a phone service, I had to pay a fee to BT for the wiring to be tested and then connected to the incoming service.
On installation of FTTC, the preferred arrangement is to have the master socket located next to a desktop computer, so the original wiring arrangement often needs changing to achieve this. It means that having an engineer install can be very advantageous.