“Normal call clearing” is an ISDN cause code (seee ITU Q.850, referenced in Q.931 for D-channel signaling on PRIs and subsequently incorporated into H.225.0 within the H.323 protocol suite) that explains what happened when a call is terminated. This is a typical error response that means “everything seems fine” from the perspective of the control layer setting up and tearing down calls. You may hear silence because a codec negotiation failed somewhere or a routing error or firewall blocked a media stream, but as far as the call control layer is concerned the call was successful. So this message just means you have to dig deeper and look for more clues elsewhere.
If you have a gateway connected to a PBX or PSTN, then it means your call succeeded on that TDM interface but choked somewhere else on a VoIP call leg after the call setup was successful on TDM leg and probably the VoIP call leg.