Justin Bieber is a reckless, no good prankster -- this according to 2 people in Texas, who are now threatening legal action against the pop star ... claiming Bieber coaxed millions of people into harassing them over the phone.

In case you're not one of Bieber's nearly 19 MILLION followers -- the singer randomly Tweeted a phone number earlier this month with the caption, "Call me right now." But the number was missing the final digit ... leading hordes of teen girls to guess what it might be. The tweet was later deleted, but the damage was done.