I have seen this sign hundreds of times. It says:
24 Hours Hot Line
That’s a perfect example of a plural noun being used to modify another noun, like “cutleries station”, except that “hours” is a legit plural and “cutleries” is not.
I think the sign should say
24-Hour Hot Line
because I think it’s better to modify nouns with singular nouns, even when there are twenty-four of the noun in question.
On an unrelated note:
I don’t know why Singapore phone numbers often don’t have hyphens where I’d expect to see them. I think we Americans pretty consistently put a hyphen in 1-800, and we put them after the first three digits of a seven-digit phone number, so I always expect to see one after the first four digits of the eight-digit phone numbers here. Sometimes there’s a space, sometimes there’s a period (“full stop”), sometimes there’s just nothing.