The Conduit Dialer is a large console of the Atlantis hub used by Michael Ford to select the mission he wants to go to. The dialer can choose all missions that do not take place on Atlantis itself to be replayed at any time after being beated for the first time.
After Ford comes back from his mission in Washington, D.C., the dialer stops working because of not been maintained over the centuries and he has to reactivate it by going to the depths of the ship.