In this scenario an established call is made and connected, after which the call goes directly to a fast-busy signal. There is no evidence of break-up or choppiness, or fading.
The loss of audio, especially outbound audio, can be caused by insufficient bandwidth for the RTP data. It may come back with periods of choppiness or broken voice, which is a further indication of not enough bandwidth or other applications grabbing the bandwidth. Dropped calls can also be caused by routing issues with voice data over the WAN Internet traffic on the ISP or other peering networks.
Making a call and getting a fast busy right away or an intercept message from the provider that the number does not exist (it may or may not be a valid number) or can't be made, means that the call was never established and could not be completed for one of several reasons. Always as a first step, check the number and redial.