I think if it is clear that the cards are being tossed into the center of the table like that face up without a verbal declaration of call, then I would likely treat it is an intention to muck, and muck the hand most of the time.
Practically speaking, however, there may be situations where some doubt may arise as to whether the player actually said call (e.g. Noisy room, dealer not paying attention, announcements going on, etc.), and I wouldn't want to be in a situation where the hand gets mucked and the player is adamant he said call. I have seen situations where the dealer mucked a hand in this situation, but the player insisted he said call and everyone at the table confirmed it - apparently only the dealer didn't hear it and mucked the cards so fast that there was some doubt as to the actual hand. Ugh.
Anyways, I personally have no problem simply confirming with the player "fold?" before mucking the hand, or saying "fold" aloud and turning the exposed hand over - giving the player the chance to correct it before mucking the hand.