Step 1: Unplug all phones and equipment from all telephone sockets including:
• ADSL modem or router
Step 2: Make sure there is nothing left plugged in.
Step 3: Plug a working phone (not cordless) into the main phone socket (this is usually where the phone line enters your premises).
Step 4: See if you can make calls from it. If you can make calls, the fault is likely to be with one of your pieces of equipment.
Step 5: Plug in your pieces of equipment one by one. Make a test call each time you plug in a piece of equipment. This will enable you to identify which item is causing the problem.