This is just a thought but the problem might be with your splitter (the device which is connected to your RJ11 telephone jack, sending high frequency input to your modem (Net connection) and low frequency input to your PSTN devices (e.g., your landline).

Splitters can fail but they are usually generic so you could borrow a splitter from a friend, plug it in, and see if it works. If it does work your current splitter is probably dead.