Not all homes were ever fitted with an NTE5, so engineer should have one on the van, and be ready to install it, hence regularise the installation.
Get them to ignore any extension wiring, just put in the master socket, and make sure that that works. Then you are free to install any phone extensions that you need from its faceplate. Also avoids any side effects from bits of old wiring you have not found yet.
Make life easy by identifying where you think the master should go, most likely replace the first junction box inside the property. Move furniture of the way etc