Landlines would not have made a difference, I think.
Fibre optics can handle a lot more data than the copper lines of landlines.
The issue in that situation would be available manpower. One person handling calls will handle the same number of calls, whether on a landline or cell phone.
Fiber optic is on our landlines, unfortunately, now.
I believe real mechanical lines handle this better. I think cellular will crash before real live wires, everything else being the same. But who really knows right now with the changes on-going.