I have seen this with many diffrent phones friends have brought to me to fix. It is either a broken antenna wire or the Tx/Rx (transmit recieve) circuit is fried.
In your case it is probably a broken wire as the system usually fails completly if the IC (integrated circuit) is toasted.
The other thing that causes this is the base unit not being powered. If it is unplugged you will get this message.
Please post any other concerns here.
Best regards, XXXXX XXXXX Smithy
Then you have an antenna wire that has broken off. Common problem with just about any portable phone.