Can you just call an all in bet in no limit Texas Hold'em if the player has less than the big blind after the flop or do you have to bet the big blind amount?

When a player is faced with a current bet amount that he has insufficient remaining stake to call and he wishes to call,he may of course fold, he bets the remainder of his stake and declares himself all in. He may now hold onto his cards for the remainder of the deal as if he had called every bet, but he may not win any more money from any player above the amount of his bet. In no-limit games, a player may also open the betting by going all in, that is, betting his entire stack.