Law enforcement sources tell TMZ ... a man called Hollywood PD Sunday threatening to blow the famous L.A. landmark to smithereens with pipe bombs -- that is, unless he was paid a WHOPPING $10,000 (insert Mike Myers pinky move here).

Big claims for big stakes, we suppose.

Here's the problem ... we're told the dude didn't even dial the right department. In attempting to reach L.A., our sources say he actually dialed cops in Hollywood, FL.

Unclear if he realized he had the wrong number, but in any case ... we're told the Florida HPD hit up their L.A. counterparts to let 'em know what was going on, nutty as the whole thing seemed.

We're told LAPD checked it out, and found no credible threat at all. Our sources tell us there've been no arrests, but the case is under investigation.

For context, the Hollywood sign is under 24/7 video surveillance, so it's always watched closely -- and there are additional safety measures in place if anyone tries to tamper with it. Of course, some have been successful getting to it in recent years ... but it's no easy target.

Just another day in the City of Angels ... not to be confused with Florida's so-called "Diamond of the Gold Coast."