Too Long; Didn't Read
Twilio's recent bidirectional streaming feature allows you to receive call audio while still sending audio for playback asynchronously. This was not possible before, when communication had to be unidirectional: either you could receive the voice audio but could not respond to it, or you could send audio to the call but not get anything back from it. The process can be seen in detail in the flow diagram below: The only supported format is that mysterious mulaw (or u-law or μ-law) encoding, which is aimed specifically for speech audio.