"Line in use" message on Motorola 450 cordless phones

Both my handsets have a "line in use" message; I have no dial tone and cannot make or receive calls (incoming calls go straight to VM). I have no other phones or faxes using my land line, but I do have DSL internet, which is working fine. I did try unplugging the cordless system and trying to use my old "plug-in" phone, but it's as if the line is still in use and there's no dial tone.

I've tried unplugging the charger and removing the batteries for a few minutes and then plugging it back in; the problem is still there.

Any help would be greatly appreciated!