Echo on other end of softphone with no mic

[wizard]

I have an Asus motherboard running Windows XP with Realtek audio onboard .I noticed that if I do not setup the inputs on the record and playback panels correctly, feedback occurs which causes the person on the other end to hear an echo when they talk even if the microphone is disconnected.I am using the softphone on a computer connected to a Motorola SB1501N cable modem.For the audio to be setup properly I select the mic volume slider on the record panel and mute everything on the playback panel except the master output slider.(On my Creative audio sound card the wave slider also has to be moved up on the playback panel to hear softphone audio.)To access these panels you select the Windows control panel, select sound and audio devices ,select the audio tab and and click on the sound playback and sound recording volume buttons.When the softphone output volume slider is moved it moves on the master output on the playback panel and when the softphone mic volume is moved it moves on the mic volume on the record panel.
If I misconfigure it by selecting the stereo mix on the record panel and the mic input on the playback panel I create 3 faults: I can hear myself talking in the headphones, the mic level slider on the softphone will no longer change the audio level heard by the person on the other end and they will hear an echo even if the mic is disconnected.I hope my experience can help others remedy their echo problems.