In reverse of the order your questions were asked:
Comment 33(c)-1 indicates that if you are responding to a consumer's error claim after determining that no error or a different error occurred, your have to provide a written explanation of your findings. Otherwise, your notice that an error occurred as described by the sender can be written or oral.
As for your first question, oral notice to the sender is sufficient. In fact, because it can be speedier than a written message via nsail mail, the oral notice is probably preferable.
John S. Burnett
Professional Compliance Nerd since 1976
Bankers' Threads User #8