Call customer care, they should be able to change the one number.
Also, in terms of choosing the number, some reps/dealers will give you a list of numbers to choose from, while others won't.
To pick a certain 4 digits or complete number, ther MAY be a personalized number charge initially and/or monthly.
I recently changed my number(to many wrong numbers, telemarketers). I was wanted to keep the same area code and prefix (1st 3 digits) , but was told no numbers were available.
2 months later, we added a line for a family memeber, and the prefix was available. When I went to change my number, I was told of the "fee". For me, it is not that serious!!!