Why it whistled
When connecting, modems used tones to “negotiate” the speed and transmission method. Whoever was online blocked the phone, and nobody else in the family could make a call. You paid by the minute, so people downloaded at night.
ADSL
Around 2000, ADSL technology made use of higher frequencies on the same line. Internet and phone worked at the same time, and the connection was always on. Today it is being replaced by fibre.
The modem today
Today the word modem often refers to the box from your provider (a fibre ONT, a cable or a 5G modem). Behind it there is usually a router that distributes the internet around the home.