line in use

Within the last few months, my cordless phone sometimes displays a "Line in Use" message when the line is NOT in use and all of the handsets are hung up. When the message appears, I can get a dial tone to call out but cannot receive calls (they go to voicemail). Also, when the message displays, the handsets connected to that phone (it is a 2 handset system) discharge as if they are being used for hours even though they are in their charging cradles. The phones are in mint condition and rarely used. I have read many articles on this problem, and they say that after performing the usual troubleshooting steps of checking that all the handsets are properly on their cradles, checking whether any other devices may be using the line, and disconnecting and reconnecting the phones from the power source, to check with the service provider so they can check for any line faults. That is what I am doing in this post. Can someone please help?