Whenever I call my wife on her mobile, the first time it always goes into voicemail. On the second attempt I invariably get through, its completely reproducible as long as the time between calls is more than a couple of hours.
She called Swisscom and they couldn't understand or explain why this was happening. She has also recently updated her phone from an iPhone 5S to a 6 and the issue is the same regardless of which phone her sim is in.
My wife had this problem with her Iphone and took it to the apple shop and they did something and it was fixed sort of, the problem reoccurred and one more trip to apple shop they did something and it was fixed. I think she got some automatic upgrade which created the problem.
The fact that the issue carried over from the iPhone 5 to 6 might be due to the fact that the phone was restored using iTunes (i.e. copying across settings from old to new phone). I might try a fresh restore of the phone, and manually copy across all her email settings, Apps etc.. I was just hoping there was an easier solution.
Many iPhone users have complained about this issue and most of them has the solution by turning off do not disturb option on the phone and indeed if you did change phones but still copied the settings then this specific setting was probably copied over. it is all discussed on the page discussed below