why my samsung galaxy s7 edge call log history is deleting itself ??

[AC Question]

Every day I have pretty inaf incoming and outgoing calls ,about 50-150. And call log history is very important for work. but when I wanted to find some older call i noticed, that call log history every day is deleting its oldest calls ?? what is capacity for call logs? (like for example yesterday (08/11/2016 first call was made on 26/10/2016, but today it changed two times and it shows calls starting just from 01/11/2016)