The G4 version of the Ericsson is the one that could be connected to the phone lines.
It is a FXO device, one will take battery from a external source.
There is another version that is the FXS, the one that could be connected to the phone.
With a FXO device on one end and a FXS on the other, you could extend existing phone lines.
The quality of the ericsson is excellent, and you only need about 30 minutes to setup a link