Why is the speaker on the phone not working?

The cable from the base to the handset may not be properly connected. Check the connection on both the handset and base to ensure the cable clicks in. If this does not resolve the issue then you will have to open the device and ensure the connecting wires for the speaker are intact. Soldering will be required to reattach the wires. The speaker may need to be replaced if none of the above interventions work.