Could be several things.
1. It could be something else connected to the line. Either try a different phone in the same socket, or take that phone to someone else's house and try it there. Just to confirm the problem is in that phone.
2. Take some photos of the inside of the phone. Include a closeup of the circuit board.
To attach photos to your post: Each photo should be in JPG format and no larger than 140 kilobytes. Click the Go Advanced button, and click the paper clip icon in the toolbar above the text area. On the Manage Attachments dialog, click Browse, select the file, and click OK. Repeat this for the other files. Click Upload. When the files have appeared in the Current Attachments area, scroll to the bottom of the dialog and click OK.
3. Do you have a multimeter?