Question [Q] Contact Default Number

Hello folks,
I have Galaxy S5 Running the Original Samsung Firmware 4.4.2.
Since I bought the phone I have a very annoying issue of fixing the default number for contacts with several ones.
Whether from the contact application or from the popup screen when u click the icon of a favorite contact, I fix the favorite number, I use it, few times in a row, it works, but then some moments later, it loses it !!! so another popup comes from the favorite screen or the first number in the list is called instead of the default one.
Does anybody have this problem? Any solutions? Or is there smthg I'm doing wrong?
Thanks for your help.