I currently have 2 line with ATT and when I went through the 2Line signup. It assign me a new number for both. There was a dialog box that said something about being able to switch back to your old number but did not have any info on how long it will take.
Were you assign a new number for you existing ATT line when you signed up?
You should sign up using the existing numbers - not your address - and we'll switch them and keep the same wiring, all at once.
Cases where this is not possible include numbers on Comcast and U-Verse.