Time Complexity of an If Else Statement

I am trying to learn about time complexity. I have done some research on the step method but am not sure how to call use it with a IF ELSE statement. The problem is below. I think the time complexity is O(n) but I am not sure.

The function is given is below with some of my //notes/ thoughts Any help in figuring this out would be greatly appreciated :)

FUNCTION fibonacci(number)
 IF number < 1 THEN    // constant operation, will only run once
   ERROR
 ELSE IF number = 1 or 2 THEN //constant operation, will only run once
   RETURN 1
 ELSE  //these only run if the number isnt one or two so it could be N times- but not sure?
   CALL fibonacci WITH number - 2 RETURNING twoBack
   CALL fibonacci WITH number - 1 RETURNING oneBack
   RETURN twoBack + oneBack
 END IF
END FUNCTION