Transferring ATT number to Google Voice?

I know its possible,  but just wondering if anyone has done this and as such as any tips and tricks to ensure the process occurs without a hitch. 

For quite some time I've been wanting to get a new cell phone number to be able to screen out old callers, yet I dont want to totally lose access to my current number as I've had it for a super long time. 

I know its possible to transfer it to google voice, the part I'm un sure about is how to ensure that my att line doesn't get canceled in the process as normally when a company receives a transfer request the old line tied to it is automatically canceled. 

Also when getting a new number, is it possible to have a say in what new number is assigned or is this just completed automatically?