Some of us remember when it took more than a decade to get a phone — and then it was only a party-line version. State service was always "take what you're given when we choose to give it to you", and too many of the state staff made clear what a favour they were doing for you just by doing anything at all. There seem to be some survivals of these attitudes in various ex-state businesses and in the Labour sacred cow health service.
Normalise for real GDP, now make sure you are comparing it to getting a brand new line infrastructure installed.
It takes weeks or months for the private sector operator to do anything if it is more sophisticated than simply pressing a button on a computer.
Also note that the GPO/BT would be much less likely to simply tell you no if you asked for a phone line, rather than put you on a waiting list.
Go ask for a new phone line or fibre link out in the middle of nowhere and they will either just outright refuse or quote such a hilariously large amount that it is effectively saying no.
EDIT:
Oh and remember that the GPO/BT was growing the number of phone lines rather more rapidly than private operators would need to.
Between 1970 and 1975 the number of households with a phone line went from 35 to 52%.
That is an extraordinary growth rate, far higher than anything achieve in the privatised era. Because by the time private BT came along the rollout of telephone lines was functionally complete.
By 1985 it was above 80%.