Sosa

Check my mail and I rarely look at the envelopes to see if they are addressed to me. So I'm going through this cell phone bill and was like wtf mine usually isn't this high, then look it over more and it isn't even mine lol. Someone else in the town.. What do I do? Call the cell # and tell em I got their bill or just take it back to the post office even though it is already opened up.