He used a little plug in splitter. The pros on the HSI forum tell me he should have installed a new jack in the wall that has two plugs instead of using the splitter on the exiting jack. It's not a signal issue or anything like that- it's just a physical potential point of failure.
When they installed my service, he used a splitter on the existing coax cable. That cable went to the modem. There's a telephone jack on a little beige box on the wall that also plugs into the modem for the phone.