I started working for the UK Post Office Telecommunications Department (latterly BT) in 1975. We didn't really have formally defined availability targets such as Five Nines or who knows how many Sigmas. Or none that I know of. The rule of thumb was that if you didn't get dial tone by the time you put the phone receiver to your ear their was something wrong.
That lovely sound reminiscent of someone grinding dog food meant that the system was ready for your dial pulses. God forbid that there was silence; that would require you send your servant to deliver a letter of rebuke to the local Postmaster.
As for having Customer Services? Yes we had many people working in Customer Services.We had lots of highly polished turds.