According to TDA 51 B, in this situation the TD can either hold the player to the full call or allow them to forfeit the chips they put in and fold. I know this is not popular on this forum, but this would be the correct, standard ruling in a tournament. Of course any cardroom can have their own rules but they tend to follow TDA.
Quote:
51: Binding Declarations / Undercalls in Turn
B: A player undercalls by declaring or pushing out less than the call amount without first declaring “call”. An undercall is a mandatory full call if made in turn facing 1) any bet heads-up or 2) the opening bet on any round multi-way. In other situations, TD’s discretion applies. The opening bet is the first chip bet of each betting round (not a check). In blind games the posted BB is the pre-flop opener. All-in buttons reduce undercall frequency (See Recommended Procedure 1). This rule governs when players must make a full call and when, at TDs discretion they may forfeit the amount of the intended undercall and fold (see Illustration Addendum). For underbets and underraises, see Rule 52.