In a tournament, if one player goes all in and another calls (and there are no other players in the hand), all players still in the hand have to reveal their cards right? I was in a situation where one player went all in and the other called on the river and the player with the losing hand and one other player not in the hand said the losing player didn't have to show their cards. That doesn't seem right.

All in and a call preflop: Both show
All in and a call post flop: Both show
All in and a call on the turn: Both show
All in and a call on the river: loser doesn't have to show?

My understanding is that any hand that goes all in or calls an all in has to show when there's no more action possible or at showdown if they make it that far.

You're wrong about the others. There's no need for both to show at any stage - you must only show to claim a part of the pot (for which all cards must be shown). AFAIK this applies equally to cash and tournaments, but etiquette dictates that you do show to speed the game up. Of course, you can invoke IWTSTH, but that makes you look like a royal *******.

In tournaments cards do need to be turned up, although I'm sure some tournaments can make their own rules. This is from the offical TDA site -

11: Face Up for All-Ins
All cards will be turned face up without delay once a player is all-in and all betting action by all other players in the hand is complete.

I don't see anything specifically about the river.

Losing hand is supposed to be turned over. It's to prevent things such as chip dumping.

That my friend is correct!
To prevent chip dumping in tournaments any all in and a call situation with action complete and no other players in hand, both cards from both players must be face up

In live tournaments cards do need to be turned face up. However, when the all in bet and call is on the river and the winning player exposes his hand first, quite often the losing player attempts to muck his hand. Any player at the table may request that his hand be turned over if requested before his hand hits the muck and the dealer should expose his cards.

I can see why this would happen though. on the flop or turn, there is still so much to have happen, but on the river, all in and a call then the card is dealt and very quickly the winner is shown.

the TDA rules are usually what most casino's follow isnt it?

its wierd what floor men can say. I was in a cash game once in a canadain casino and i rivered a fullhouse. I shoved and get called and show the winner, for a 1800.l00 pot. then some idiot that was a friend of the villian, says, hey dealer you didnt burn a card.

long story short, they turned and burned and i lost and i lost the pot and all my money in front of me and I was like WTF is going on here? the floor man was serious about his decision and wouldnt back down and I freaked. Then I look over at the wall and right there on the wall is a sign with rules and specifically what to do when a burn card is forgotten and the action is given back to players and hand is dead.

So, i actually came out ahead, however, it took the actual manager of the entire casino to come in and rule on it, because the floor man thought even though the rules were on the wall, once he made a decision it should have stayed. I think he was fired. Havent seen him again.

so....sometimes it gets mixed up , its good to know the actual real rules of whatever casino your in.

Rules vary from room to room. but generally in cash games you don't have to show and in Tournament you do to avoid collusion problems as much as possible.