Is there any way to make silence detection on start of the call?

Like answer a call, playback a ringtone back to legA, and meanwhile try to understand, if legB is totally silent. If it is silent for, let’s say 5 sec - hangup the call.

One of ways to fight spam machine calls (yes, sometimes it looks like DoS attack)

Right now only thing I can imagine - record legA to file and than analyze recording for silence.