The Senate Commerce Committee is holding a hearing Wednesday on placing unauthorized charges on telephone bills. Tuesday the Federal Communication Commission proposed new rules to make it easier to detect and challenge mystery fees on phone bills. The proposed FCC rules would require landline phone companies to place third-party charges in a separate section of phone bills and notify consumers whether they offer the option of blocking such charges. The FCC estimates that as many as 15 to 20 million American households have mystery fees on their monthly landline phone bills every year.   Witnesses: • Lisa Madigan, attorney general, state of Illinois • Elliot Burg, assistant attorney general, state of Vermont • Susan Eppley, Decatur, Ga. • David Spofford, CEO, Xigo LLC • Walter McCormick Jr., president and CEO, U.S. Telecom Association