Problem is persisting even after upgrade from WP8 to WP8.1. I tried setting ringtone from custom MP3 to some system sound with no result and now I wipe the phone. Still no luck.
Sorry for my english, hope you understand the problem and I am looking forward to your suggestions how to deal with the situation.
Thank you, Míra.
Hello. I recommended father Nokia Lumia 925 and now he's expriencing some weird bug. Sporadically when incoming call should trigger vibrations and ringtone the phone makes only clicking tone and that is all. No vibration, no ringtone. Only the display is signalizing incoming call. As you see, it is kind of problem when you have your phone in pocket. As for me I am android user so my knowledge of WP are limited.