Speakerphones...no feedback...how do they do it?
Cordless speaker-phones use half Duplex so the "loudest talker" controls the direction of audio, thus preventing feedback and echo.

Skype and telephony networks use a white noise or spread spectrum signal to equalize the amplitude and phase of the distorted channel so that the active mic "Side tone" can be cancelled out in the network when making a long distance connection.  this is a short test just prior to a connection being made.  ( You may recall Faxes and Modems sound the same with test tones then white noise before making a data connection 

But higher-end conference room phones use full duplex analog with high quality microphones so that when the impedance is balanced the energy from the mic is cancelled using a differential amplifier with phase inversion and amplitude adjustments to cancel the local signal or the received signal from being echoed back.

The conditions for oscillation or feedback howling is 360 deg or 0 deg phase shift and gain >=1 at any frequency.