Remember 9/11?
Even cell phones - and those were not high-power “smartphones minicomputers - clog up the lines and can’t get thru.
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.