I also have this issue with my 6s, all my contacts frustrated as I am not able to reach, I thought its service provider problem but even after changing service provider this issue still continuing, reset all settings just now, now the repeated calls button checked ON by default, seems this is the workaround, as I could test it by locking and after few min tried with other phone, the first call dropped but the autorecall could connect, saw one missed call notification still as the autorecall on other phone was set without time delay it dialled back. I see so many coming up with the problem faced by them on this matter, hope Apple support will come up with some solutions