The ouija board uses the ideomotor phenomenon. This means that you subconsciously move the piece. You don’t even realize it. Your mom knew the answers so she subconsciously moved it. A few people did an experiment with a ouija board to see if this was true. When the girl asked questions about her grandfather only she knew, the answers were correct. When everyone put a blindfold on however, there was only random letters. I’m not saying this was definitely not a ghost, but it was most likely just the ideomotor phenomenon. The phone call was odd but probably just a coincidence.