From what I recall from the 1970's, we paid a monthly charge for each line (number), phone rental (each), and then long-distance calls were itemized. If you had two phones in your home for the same line, you'd be charged an additional location fee, as well as the rental for the second phone. If you had two numbers (like one for the parents and one for the teenagers) then it was 2 lines, 2 phones, plus any long-distance calls each month.
By that time their billing was computerized, so the monthly statements would list each long-distance number you called and how many minutes you spent talking. I was calling BBS's at that point (300 baud acoustic coupler hotness!) and I had a phone budget that I had to stay under or pay the parents. Long distance calls (basically anywhere outside your city) were expensive!
Bell planned for the 2 line scenario, as your wiring out to the pole had red/green (tip + ring for line 1) then black/yellow pairs (tip + ring for line 2). So they could offer two lines without needing to run additional wires to the house. Or use the second pair if the first one went bad. The Bell System did forward-thinking like this all the time. And could afford it because (as mentioned) they amortized their system investment over 40+ years.
By that time their billing was computerized, so the monthly statements would list each long-distance number you called and how many minutes you spent talking. I was calling BBS's at that point (300 baud acoustic coupler hotness!) and I had a phone budget that I had to stay under or pay the parents. Long distance calls (basically anywhere outside your city) were expensive!
Bell planned for the 2 line scenario, as your wiring out to the pole had red/green (tip + ring for line 1) then black/yellow pairs (tip + ring for line 2). So they could offer two lines without needing to run additional wires to the house. Or use the second pair if the first one went bad. The Bell System did forward-thinking like this all the time. And could afford it because (as mentioned) they amortized their system investment over 40+ years.