On a three phase line phase to phase voltage is 1.732 times higher than phase to ground voltage on a grounded system. Also each phase is 120 degrees out of phase from the other phase, so two phases will not be at the same potential. Many of the California distribution systems are corner grounded Delta 12470 volts. So phase to phase you measure 12470 volts. Phase to ground or grounded conductor will be about 7200. Delta distribution SUCKS. It has some advantages over WYE systems that are more prevalent now. But WYE has better voltage regulation and over current coordination.
Broken overhead cabling is not common. It's typically burnt down, line faults, trees, trees, fucking trees. Animals also cause a bunch of hell. But for line to just come down it is rare many other factors contribute to that. Especially in wind storms when stuff is picked up and carried to the lines and structures.