A cordless telephone-to-sound card interface adapter provides a connection between any standard cordless telephone system and a sound card of a computer for PC-to-PC or PC-to-telephone voice communications over the Internet. The interface adapter includes a hybrid transformer circuit for interfacing a two-wire circuit of the cordless telephone and a four-wire circuit of the computer sound card. Consisting of passive components, this hybrid transformer circuit cancels out the voice echo that would otherwise exist due to mismatched impedances between the two media and/or from leakage of the signal being transmitted and received on the same pair of telephone lines. Preferably, a Universal Serial Bus (USB) interface is provided to supply a bias voltage to the cordless telephone system through the interface adapter. The interface adapter may also include interfaces for connecting with external computer speakers and microphone, and a control circuit for automatically switchably coupling the computer sound card to either the cordless telephone or the external speakers and microphone as appropriate.