SER is not a B2BUA. It mostly acts as a proxy server. This means it is primarily designed to scale and handle heavy traffic. It does not keep call state and therefore it is unaware that calls are even in progress.
In other words, SER is completely oblivious to the fact that calls are connected. So when SER connects a caller to a callee, SER forgets immediately about the call. By doing so SER can handle large volumes of subscribers. The bad news is that SER cannot disconnect a call or otherwise participate in the call.
If you need this sort of functionality then you need a B2BUA so act as a "middle man". This is very common for prepaid calling card applications. By using a B2BUA, the caller and callee never communicate directly to each other. Instead they communicate only to the B2BUA, which is aware of call state.
Credits: This information was adapted from contributions from Jiri Kuthan.