I am not sure how flexible solution you are look for but you can simply use substring() to get the first 6 digit from sentence.
string text = "123456 Mr. Williums, 330514 Lovebird Lane, Phoenix. phone: 333-4444, Fax: 313-4141";
string firstSixDigits = text.Substring(0, 6);
If you want more sophisticated solution then you can use regular expression.