do you use MGW?This issue can be related to core too.
try to find IP of calls which face with Silence and make sure there are appropriate connectivity between IP of MGW’s. may be you send properly but another MGW can not sent to you, due to Routing problem or faulty board. then Silence happen.in this case usually after some second call will be released.
Also you can controlling ABIS interface by checking BRPB board and A interface with DRTB the tools name is DSP Insight- But I don’t have any practical experience with ZTE.