Other mistake: When The Chipettes perform their audition for Ian Hawke they are standing on the Jett Records sign on the roof of the Jett Records building. In order for the sign to read correctly from the street, Ian and The Chipettes should have a backwards view of the lettering. In the movie the sign reads correctly from Ian and The Chipettes point of view, meaning that it is giving a backwards view to the street, ie. it is the wrong way round.
Other mistake: The code for the lock on the cage near the end of the movie when the the girls are trapped in the cage is 777, not 111.
Answer: Alvin could have easily known Toby's number and told it to her. Ian just simply put down his phone not even thinking that the chipettes could get at it from inside of the carrier.