well it really depends what type of problem you have : the voice you hear tends to fade away, disappear, get noisy, sound metallic ? In this case, it is probably interference :
– either in the UL of the cell where the called party is located
– or the DL of the cell you are located
on the QOS statistics, you should see HO due to UL quality, or DL quality, and also (possibly) interference HO (intracell).
If you just don’t hear anything (no voice at all starting at the pickup), then it is a CIC definition problem, or (more generally) a routing issue at NSS side.