Ending question...

mmhh...
Not so obvious...I don't like trying to explain everything...
Anyway, I think that neither Jean george nor anne changed the code. In paris codes change randomly so that people can't get in, the inhabitants reveice the new code by mail, but they do not change it themselves! How do you want the other inhabitants to get in if anyone could change the code when they want to?

What letter did George write Anne?

I just finished watching the film so this might be a little vague...but i think that what haneke was trying to show was how different cultures have a code, to keep their members inside, and leave the others outside. People who know the code will always know how to enter, will have a place to belong and remain. George who has remained in a diff. country has either forgotten the code, or it has changed behind his back. He will eventually be initiated into the code (dinner with friends, subway rides, shopping with your girlfriend), but for now he has to stay out into the cold.

another idea on the subway scene at the end : the rage shown by the boy could be due the lack of communication shown by anne and the rest of the people on the train(although anyone would have ignored his behavior). He wants a reaction, and the only way he can is when he turns violent.

to me it seemed like he went into the perfume store to get a gift for anne. in his letter his apologized for how he acted previously. he goes to the door and the code is changed. i think this is a sign that she changed the code to lock him out. by showing him buying the present he shows us that he assumes he needs to do something to get back on her goodside. the only reason to show us this is to imply that she has locked him out. without him buying the gift we could interpret it differently, but the gift adds context that, to me, shows she has changed the code. hence the title (obviously it runs deeper with a metaphor for interpersonal communication and communicatin between different cultures as well.)
Although I do think there is something to what the previous poster said in regard to her not being able to hear the phone in the bath. he would not have included this line if it was not important. perhaps she did take a bath after getting spit on and thats why she didnt answer the phone. and perhaps in reality she changed the code cuz she felt vulnerable. with her man away, being alone, i can understnd why she would feel vulnerable.

perhaps the bath that may be implied by her not answering the phone could be an attempt to wash away the guilt she has from not doing anything about the letter and allowing the little girl to be killed. im not sure if this scene in the movie happened before or after the note and the girls death. either way even if it happened earlier in time, haneke could still use the bath for this metaphor even if the character herself at the time did not know about the note or the girl. the important thing is the viewer knows this.