If you are talking abot Ericcson MGW – you should use GUI for MGW connection, and there is an option like ‘Trace call’. That is only tool for call tracing in MGW. ctrai can be executed only at AXE(MSS or MSC for example)
If BSC also Nokia, check CCSPCM(CIC) on both side, and transmission way. Also it can be related with active feature Soft Channel Capacity on BSC. And lastly check parameter NBL on BTS’s in non-CommonBCCH cells must be equal 0.