YES, the CapTel 840 works with just one telephone line as well (1-Line mode). Using 2-Line CapTel is purely optional. You can still make and receive telephone calls with captions with 1-Line CapTel. The main different between 1-Line mode and 2-Line mode is in the way that incoming calls get captioned. With 1-Line mode, callers need to dial the captioning service first, then enter your telephone number, in order for you to see captions of the call. With 2-Line CapTel, you can turn the captions on and off at any time, on any incoming or outgoing call.