When all the numbers required to win a prize have been marked off, the player shouts in order to attract the caller's attention. There are no formal rules as to what can be shouted, but most players will shout "yes" or "bingo".
State the letter first, followed by the number, such as B5! It's helpful to say a word that starts with the letter for letters that sound similar to other letters, such as B and G. For example, if the call is B10, you might say, B10, B as in Bingo!