Factual error: In one scene, Mrs. Doubtfire pulls the symbol off of Stu's car. She shouldn't have been able to do that, since a cable connects the symbol to the inside of the car.
Factual error: The piano was invented by Bartolomeo Cristofori in the year 1720, in the 18th century. Miranda recommends a "17th-century grand piano" for Stuart's bed-and-breakfast inn.
Answer: Daniel was pretending to be different people that were just all very horrible, so he used different accents. This way, when the "British nanny" calls, he was hoping Miranda would instantly consider him. If he called pretending to be Ms. Doubtfire right away, she might not have basically given him the job right away. And if he used the charming British accent for all his characters, it would have lost its charm when he decided to be Ms. Doubtfire.
Bishop73
Basically this. He wanted Miranda to be frustrated by the other calls, so that Mrs. Doubtfire would seem like a refreshing change.
Azalea