Our group were given a YT2000 with the Remote Calling modification fitted to one of our HP's.
We are unsure how this works. Do we need to do any checks or just press a button on a remote that our pilot PC carries?

Do we need an on board Autopilot Droid Brain or a Slave Circuit? could someone explain these to me?
We do have a NPC astrobot with piloting ranks, the unit does stay on the ship for most sessions but not always.

I hope you can help ;o)

I'm not sure what you mean with "Remote Calling". Is that an attachment, or are you thinking about the piece of equipment from Suns of Fortune? The latter is merely a remote control and doesn't require a hard point on the ship in itself. The latter would require a Daunting: Piloting (space) check to steer the ship to the caller's location.

Other than that I think it's pretty much up to the GM how it gets handled. Personally, I'd run it so that the ship would come automatically to the caller as long as the trip didn't involve any fancy manoeuvring with lots of obstacles. For more sophisticated control you'd need the Autopilot Droid Brain attachment to actively steer the ship with a Piloting (space) check. If you have the Slave Circuit attachment the pilot can use his own skill to pilot the ship from afar (with the appropriate mod, IIRC).

Of course, if there's a lot of signal interference like jamming or other background "noise" it would be appropriate to have the pilot make a Computers check when using the caller in order to get through to the ship.

The droid pilot brain is also 0 hard points, but has a base piloting of 2 (can be modded to 4). While the remote call on its own is akin to flying the ship as a drone with out the video camera, the droid brain can actually fly the ship on its own and can possibly provide skilled asistance to the pilot.