I like my rats to use a litter tray because it keeps the rest of the cage much fresher, and because they will then learn to use a tray when out and about in the room. Some owners find it useful to use a different substrate for the litter trays, but I've had success using the same substrate for the whole cage.
Put the litter tray(s) at the back of the cage, in the darkest corners, as these are the places the rats will tend to use even without a tray. If you find any poo outside the tray, move it into the tray. It can help to give the tray a quick spray of a strong smelling substance such as perfume, as this seems to spur the rats on to try and cover the smell with their own. Once you have trained your first group of rats, any others added to the group seem to be trained by the incumbent rats.
If you're hoping to use a tray during freerange, again put it in the darkest corner of the room and follow the same process.
Rats can be trained to recognise their own name and to come when called. The easiest way to gain a rat's attention is with food, as most rats are very food oriented. Use something small such as a tiny piece of bread, or even a piece of food from their own ration, and say their name whenever you give them a piece. It won't take long before they get the idea.
I recall my groups from freerange by feeding them their mix as soon as they come back. I use a small handbell to call them, and it's very amusing to watch them file back up the ramp into the cage. Shaking a food pot or crumpling a plastic carrier bag are other noises that carry well for the rats and get their attention well.
Rats are very capable of learning to do tricks, and although I haven't attempted this with my own I've seen some very impressive videos online. They can also be clicker trained.