Well tbf that was because of capacity, if you ve a small duct with a 1000pr cable in it and 10% are faulty, and the cable is 85% utilised plus the main exchange as no more relays for switching, it takes a great deal of time to provide a line.
Shared lines? of course and dolly board operators, physically moving one line to another, a public service organisation based in Reading was still using this in their head office up until the 70s....
Digital Exchanges, like System X, fibre and then IP total transformed capacity and the mtce of lines and equipment.
These advances, lead by UK companies like Plessey and GEC transformed how telecoms was provided, as muuch if not more than privatisation, which of course relies very heavily, still, on the old BT, now OpenReach networks, exchanges and skills.
People have no idea the complexity of electro mechanical exchanges, the GPO used, the skills required to keep them working, might as well blame a typist because they cannot produce documents as fast as a Xerox can.