Only couple in a year I dont think is harrssment. You dont seem too concerned that there might be some truth to that text.
They could be using the Text Me app, which is an app that generates a totally random number to text and call from.
If you have Facebook enter the number in the Facebook search field. If the have their phone number linked to their FB it will pull up their acct. I had a guy text me a very inappropriate picture, it was a wrong number. The idiot didn't realize it took me less than 5 minutes to find out his real name and that he was married and I also found his wife's facebook. I told him this. Made him sweat a little and scolded him for being so stupid as to send a pic like that to a wrong number esp when he is married.
Good luck in your seach fo the truth. You may not like what you find.