Recalibrate the capture VAD to the quiet handset voice: onset 1.4%->0.7%
FS (was never triggering -> 'no sustained voice'), silence 0.6%->0.34%
FS and end-of-speech window 600->900 ms so a whole sentence is held
instead of cut mid-phrase. Also force slic_ring_stop() on incoming
pickup so the physical bell always stops when answered (phone.c/slic.c
s_ringing could desync, leaving the bell ringing through the call).
clement