Did this ever get resolved for you? I have been having the exact same issue. People must call me twice to get me. Calling support didn't help with my problem.
We don't want you to miss out on any important calls. When did this issue begin? Have there been any recent changes on your phone? Does this occur when you're connected to Wi-Fi or our netwok?
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Is this only happening with you? Or is it all callers? How many bars does she have? If it's initially going straight to voicemail without ringing then she wouldn't show a missed call...it's as if the phone is off.
I know it's been 2 years since you posted this, but maybe this helps others...
My mother's iphone 6 plus has same issue (all callers must call twice to get through). It was the Do Not Disturb functions - most settings were on.
Settings -> Do Not Disturb -> Deselect anything highlighted
worked for her phone hopefully this helps