Why are my iPhone contacts getting lost?

From some time ago i started to notice that my iPhone 4s contacts are gone after a springboard crash,and sometimes not without a crash.I open my contacts to call somebody and no contacts at all.When it happened to me for the first time i didn't pay much attention ,i thought that it might happen sometimes.But the bad thing was that i didn't have iCloud at that first time so i had to rewrite my contacts from a backup that i had in my computer wich was a excell document.So i readded all my contacts and made a iCloud backup.But after a very short time it happened again ,and again,like at least once a week.My iPhone was iPhone 4s with iOS 5.11 installed and i thought maybe it is because of my software beeing old.But then a few of my friends told me that they started to experience the same thing happening to them too ,and they had newer software installed.So this started to look like a common thing.But what could have been the cause of that.I was like the tech guy in my city and many people were bringing their phones to me when they needed updates or some apps.So i asked them did you notice anything like when were the contacts erased.What did you do when that happened?And suprisingly many of them told me that they were trying to call somebody from the free calls and messages app (Viber) and they suddenly get a crash and all contacts lost.

So few days later i noticed that the same thing happened to me when a friend was calling me from Viber.I wanted to answer him and suddenly a crash and all contacts lost.So i think i found what did this happening all the time.I use Viber daily and i receive and make many calls from the app,messaging also.One other thing i noticed after i got my contacts back from iCloud,is that all my contacts pictures were messed up and changed places.So when my father calls ,my friends picture appears and it is so frustrating because i delete them all the time and then happens the same thing.So i decided to write this blog to share and to find out if this happened to others also and maybe i could possibly find a solution for this.So pls write your comments below and share your experience with us and maybe we will draw someones attention to find a fix for this.