How to remove alphabetic characters from a string

string a = "(570) 847—1059 Alternate";

string b = "570 847 1059 Alternate";

string c = "570-847—1059 Alternate";

a = Regex.Replace(a, "[A-Za-z]", "");

b = Regex.Replace(b, "[A-Za-z]", "");

c = Regex.Replace(c, "[A-Za-z]", "");