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Send them a DV letter.
You can also call Farmers Insurance and ask them if you owe any money and find out what is going on. If it turns out you do owe them ask to pay them directly. Also find out if they sold the account to a CA.
And if they send some oddball answer, as outlined in the link below, then the below letter should help:
http://ficoforums.myfico.com/fico/board/message?board.id=rebuildingcredit&message.id=13494
But as mentioned, send a DV first.