main causes are related to interconnexion of transcoder to MSC (A interface), or transcoder to BSC (Ater interface), as far as I know. But I’m not an expert in the field though.
From BSS QoS point of view, you can’t see any problem. You can just detect that TCH duration is very short, while there is no call drop. It means the subscriber hangs up by himself after just few seconds.
But it’s just a side effect, and if this problem is marginal then the TCH duration in your network is probably not impacted.