Alice Kriewald wants her experience with three scam artists to serve as a warning to her fellow seniors not to give money or personal information over the phone or online.

In the spring, she got a call from a man in Portland, Ore., who told her she was supposed to call Lloyd's of London with a certificate number he provided and that she would receive $450,000 when she did.

She didn't think anything of it until getting a call from another scam artist a couple weeks ago. This man told her he was with the Federal Gaming Corp. and she had won $250,000, but because the prize was in Canada, she needed to pay a $2,300 duty...