SOLVED
Okey all, listen carefully the solution. It's basically weird and Apple must solve this issue. because it's not headache of the end user to do take care of this method what I am going to explain.
It's all about your contact list. the format of your google sync account and/or yahoo contact list account sync causing some strange issue with the iphone especially when you using some calling app. what my alab fellow did and with confidant is, he erased the phone, then singed in to icloud to sync my contact list. then google contact list sync. and then erase all app even the build in one except the notes app. and then backup in icloud and erase the phone again.
after coming back home, I also did more precise things. I stopped Google and yahoo contact list sync and kept only the icloud one. I directly go to google account on website, there it provides to merge the duplicate contacts. I did all that. then export the list into vcf format and then I singed in to icloud web account and imported all contact list there. and then manually I checked each and deleted unwanted/ambiguous names/numbers. also there were some names with smiles and special chars that may cause the format conflict issue.
Believe me It's 3 weeks now and never encountered with the same issue. otherwise I already got one mobile replaced and the 2nd one had same issue for 3-4 time even after restore and erase until I did this.
Hope this thing will end this issue and Apple must take care of this contact list conflict issue.