Avoid enter any characters except numbers and plus sign

tel.js

        //<editor-fold desc="функция проверки введения только цифр">
        var tel_01 = $('#con_tel');

        function isNumber(n) {
            return !isNaN(parseFloat(n)) && isFinite(n);
        }

        function onlyNumbers(input) {
            input.on('keypress', function(e){
// check IE version from this function <script src="https://gist.github.com/mir4a/5211477/raw/b78e8f60b2cc195a7831672cd2ee0fe308f17e3f/check_ie.js"></script>
                if (checkVersion() < 9 && checkVersion() != -1) { 
                    var char = String.fromCharCode(e.keyCode);
                } else {
                    if (e.charCode != 0) {
                        var char = String.fromCharCode(e.charCode);
                    } else {
                        var char = e.keyCode;
                    }
                }
//                console.dir(e);
                if (char === "+") {
                    return true;
                } else if (char != 8 || char != 46) { // allow backspace and delete keys
                    return isNumber(char);
                }
            });
        }

        onlyNumbers(tel_01);


        //</editor-fold>