What can I do if someone already moved in to the house and I was still being billed for mortgage?

The "trustee" that I had told me that all I had to do was pay two months mortgage and he would take over the payments until someone moved in and took over payments. It was August 1st when we moved out so I paid August and September payments and ever since then, I kept getting phone calls and mail from the mortgage company harassing me about delinquent payments. I tried numerous times to make contact with our trustee and it got me nowhere. The "trustee" made several excuses about the delinquent payments. March of this year, I went to the property only to find out that there was an occupant living in the house. The occupant claimed that he has lived there since "mid September". I was NEVER contacted about the sale of my estate and I am still currently getting billed and asking about foreclosure about this property.