Crosstalk is when two wires are close together, one induces a voltage in the other (it's how transformers work). The trouble with house wiring is if you run a phone line parallel and close to an A/C line, it will induce a 60 cycle hum in the phone line, which will drive you utterly mad.
The electrician was installing the cat3 phone wires in the same holes as the A/C wires. It was guaranteed to induce a hum.
The correct approach is to keep the low voltage wires as far from the A/C as you can, and if they cross, keep the crossing at 90 degrees. Don't run them in parallel unless they are separated by feet (plural). If you have to, you can run them in grounded metal conduit as a shield.
I probably went overboard on this, but doing it inadequately meant 60 cycle humming on the phone lines and an utterly ghastly bill to rip the walls up and fix it.
He also was installing cat3 (POTS cable) and R59 cable (1970's cable TV stuff). I pulled that all out and threw it in the trash, putting in cat5 (cat5e and cat6 are better, but weren't available then) and quad-shielded RG6 cable.
Getting the best cable available cost maybe 20 or 30 dollars more, so it's crazy not to do it. The network and cable speeds have all increased dramatically since I installed it, and the cables have all worked like a champ.
Even the cat5 cables are happily carrying gigabit Ethernet.
I have a couple plugboards in the basement where all the wires terminate.
I did the whole thing for less than a grand. If an electrician had done it, it likely would have been 10 or 20x more.