hi sakuraba
Usually when it happen, it was related to A interface trunk (between TRAU and MSC). one of or may be more of your trunk is in not good quality. So when a call sieze that trunk it recieve that problem. Here i’m suggest that, you check the trunk quality, for example every MSC have one command to check the PCM quality or via the trunk traffic analysis, or make a test call per trunk basis..
hope it can give as a guide for you.