But this case is different. It is not lack of response from the UI, on the contrary, as you can see on the video, the UI does instantly respond to pressing the “red handset” button. There is no slowness or lag, it’s very responsive. After I press the Hangup button, the call window instantly closes and a system notification is displayed that the call has ended (with connection time 0:00:00), i.e. everything looks as it should. The Phone UI acts as if it normally rejected the call. But it doesn’t. It rejects the call only the second time, but even in that case some incorrect signal is sent to the mobile network because instead of being forwarded to voicemail (or busy tone being sent if there is no voicemail forwarding enabled) like it is with this SIM card in any other phone (or even in this same phone reflashed with Android), the caller hears “beep beep beep” connection error tone. Something is simply scr*wed up with modem handling by SFOS.
BTW. I’ve just entered Android settings, checked ringtone settings there and identified that this second ringtone being played simultaneously with my SFOS ringtone is the default Android ringtone. So maybe it is the Android support stuff doing all this mess.