when ever call ends dialogue appears with same ad and ok button

when ever call ends dialogue appears with same ad and ok button

I have samsung note 5, when ever call end i recieve a dialogue with ok msg.. that says to send sms to a number in order to win 100k$..
if someone calls and call end it doesnt appears.
if i call but the next person doesnot recieve it.. the dialogue doesnt appears
if i call and the next person recieves,, when call ends the dialogue appears with ok button.

the dialogue appears if internet is available or not
the dialogue appears even if phone in safe mode.

this dialogue is of white colour with simple same ad everytime..

the mobile was never rooted

pls assist..

Regards