Crossed line is where you are getting the wrong number/line to your house, you have cross talk on your line, overhearing conversations and is caused by your line picking up current from the other line. This can be caused by poor insulation of the wires, from age or dampness. As John46 says the engineer should perform a series of tests ideally from your house. Although as usual the fault is unlikely to be present when they turn up, but hopefully they may be competent enough to locate it! Capacitance likely to be unbalanced for a start and probably with battery voltage on the line. They'll probably loop your line at exchange and test from your house to get a fault measurement!