A flexible asymmetrical digital subscriber line (ADSL) transmitter is able to operate simultaneously with integrated services digital network (ISDN) terminal equipment (TE) using a common telephone line (18). The ADSL transmitter changes the frequency content of a frequency-encoded ADSL signal (104) so that its frequency content does not overlap the frequency content of the ISDN TE signal. A corresponding ADSL receiver located within a central office (CO) adapts to the changed frequency content, allowing the ADSL signal to be transmitted over the telephone line without substantial loss of signal integrity. In one embodiment, an ADSL transmitter (100) converts ADSL symbols making up the frequency-encoded ADSL signal (104) into a corresponding time domain signal. The transmitter (100) then interpolates the time domain signal and high pass filters the interpolated signal. This high pass filtered signal is then converted to analog form, bandpass filtered, and driven onto the telephone line (18).