SO FRUSTRATING!!!!!!!

Ok, so the past 2 days I have been getting text messages from this unknown number. I have no idea who it is and when i text back asking who it is they won't answer me. I broke down today and did a search for 1.95 online only to show that the name is POS- 65216....what the heck does that mean!? I know what city they are in........Arlington...and I know that their phone is through Sprint. But other then that I am clueless. And I can't block their number, I've already tried. Anyone have any ideas??? I just want to know how some unknown person got my CELL phone number! I can understand my home phone, but not my cell!! HELP!!!