The phrase 'Is that so!' is an expression of surprise or disbelief. The crossword solutions for this phrase are REALLY (6 letters), GEE (3 letters), DOTELL (6 letters), OHYEAH (6 letters), CANITBE (7 letters), OHREALLY (8 letters), NOKIDDING (9 letters), and YOUDONTSAY (10 letters). These solutions are all expressions of surprise or disbelief, often used in response to something unexpected or unbelievable.