方法一 ：根据未知替换
String subStr = phone.Substring(3, 4);
oldcellphone.Text = phone.Replace(subStr, "****");
方法二：正则表达式，
Regex re = new Regex("(\\d{3})(\\d{4})(\\d{4})", RegexOptions.None);
var test= re.Replace("12345678901", "$1****$3");