You already have a true whole number function and it works well. The only problem is with how you tested it... you've passed a numeric value to the function and +6 is not a naturally occuring numeric. The "+" is nothing more than a formatting symbol in this case.

Any conversion from +6 without single quotes to a varchar whether in your function or not, will result in just a '6'. Further, no natural numeric datatype will actually store the "+" sign so, if it's a number, you don't have to worry about testing for a plus sign.