Bell Company Coverage by State FCC Statistics of Common Carriers, 2007
This chart supplies the percentage of Bell Company lines by state. I.e., in the state of New Jersey, How much of the state does Verizon New Jersey control? There are caveats. For example, Verizon was formed by the merger of Bell Atlantic NYNEX and GTE, which was NOT an original Bell company but had properties in many other states, such as California or Florida, which are are now mostly controlled by AT&T. And this could be ascertained by cross-referencing this information starting in 1995, the year before any of the phone company mergers.
There are no other sources of this information that we are aware of, including information collected in separate states.