Verify that the computer is connected to a known-good analog (not digital) telephone line.

Verify that you are using a known-good RJ-11 telephone cable.

Verify that the RJ-11 cable is not plugged into the Ethernet port.

Inspect the RJ-11 connector and modem port for pin damage.

Verify that the RJ-11 cable is installed firmly in the modem port.

Verify that the correct modem driver is installed and that the correct connection control language (CCL) is selected. If the issue persists, reinstall the system software.

Open the bottom housing. Verify that the internal RJ-11 modem cable is connected to the modem.

Replace the RJ-11 cable/jack.

Replace the modem.

Replace the logic board.