From other comments it sounds like there are similar problems in more than one language, which I suppose is not surprising since the low counting numbers in man

From other comments it sounds like there are similar problems in more than one language, which I suppose is not surprising since the low counting numbers in many languages are short words and so have a higher chance of sounding like other short words. I suggest switching to a non-number sequence with at least the end word not being short. My suggestion: drummers, pipers, lords, ladies, maids, swans geese, rings, birds, hens, doves, partridge For extra safety, such as if the countdown is over a noisy communications channel or in a noisy environment, those words could be used in phrases so that even if a word gets garbled it will be easy to tell were the speaker is in the sequence: drummers drumming, pipers piping, lords a-leaping, ladies dancing, maids a-milking, swans a-swimming, geese a-laying, golden rings, calling birds, french hens, turtle doves, partridge in a pear tree