Echo/Feedback loop in VoiceChat over phone/speaker.

The feedback does always happen. So the question should be: Why do you not hear such loopbacks in other pieces of software. The reason is that advanced voice chat software (like skype / teamspeak) have implemented a quite complicated audio analysis of the microphone and with respect to some delay they try to subtract the speaker signal from the microphone input.

This is by far more complicated than it sounds ^^. The phase of the two audio signals have to perfectly match. So the data that goes out the speaker will arrive a tiny bit later at the mic. So depending on the room properties in respect to size and echo that time will vary. Also you can have multiple reflections off walls which makes it even more complicated.

See this post. Also have a look at this post, it's not directly related but shows some of the problems you have to tackle.