I reproduced it on the real open-source Bolna engine, found four different causes that all look the same from the outside, and fixed it. 5 minutes.
Bolna hangs up after 10 s of silence, checked every 2 s. That is your 12 seconds.
A healthy call trips it when the caller pauses. Broken voice, rejected speech-to-text and one-way audio trip it too.
The fix depends on which one it is, and Bolna's own logs tell you. Send me one execution ID and I'll tell you which.