CID NUM Alias doesn't work for internal called number

I create a extension 66766, CID NUM Alias is 6666. When I make a call to 6666, 66766 can ring, but when it answer, the number on my phone screen becomes 66766 from 6666.
However when 66766 make a call to me, I can see just 6666 not 66766. I am not sure what is wrong.