Unless they've changed their policy, you can't pick the full phone number..
You can choose the area code and I believe you can pick the first 3 numbers and they bring up a list that you can choose from..
Your best bet is when in a store tell them you want to choose your number, and probably the best they can do is say:
do you want xxx-yyyy?
no
do you want xxx-yzyx?
no
do you want xxx-byba?
etc, etc..