Why do people I don't know send text messages to my mobile phone?

Don't owe money, it's none of your business. It is likely that someone else sent the wrong number.

In this case, you just ignore him. If he keeps texting or calling you and you tell him that you have the wrong person, or that the person in front of you owes money, then use him to find you now, so as long as you don't owe money, it doesn't matter to you, just ignore him.