If the line in question is a true ISDN I can assure the Chinese were not using it to hack election machines
First, true ISDN circuits come in two flavors, basic rate interface (BRI) and Primary rate interface (PRI), BRI interfaces can handle 128 kb A second with two 64k channels and PRI interfaces can handle 1.544 mbs with 24 - 64kb channel
For a country trying to hack the USA that’s hard enough bandwidth to do anything