You will need to find out how the lines are directed to the Auto Attendant right now, so that you can change it. Start by seeing if they are assigned to the voice mail calling group.
You will use another calling group, with NO members, and assign the lines there. That group will use Time Based Overflow to hit the voice mail, and you will create a mailbox that steers the overflow calls to the auto attendant.
For the phones to ring, if they are Operators, assign the lines and set to immediate ring. For NON-Operators, assign the pool, and also set it to immediate ring.