Validate value in a Property?

Should be fairly straight forward I have a property for a phone number and I want to be sure that only the numeric values are "stored"

So lets say we have something like:

private string _phone;

public string Phone {
    get {
        return _phone;
    } set {
        if (IsNumeric(value)) {
            _phone = value;
        } else {
            _phone = StripPhone(value);
        }
    }
}

I believe I've done something similar in VB and it worked, yet not in C#?  IsNumeric and StripPhone are functions.  I can't find any examples that are similar to this so hopefully someone can point me in the right direction.