Call the local police station non emergency number where they live ! They'll go out & give the person your number, then it's up to them to call you! Tell the police it's a family emergency & it's imperative that you speak to them! My dumb mother in law did that to me once cause I was out with the kids having fun & didn't answer her calls all day! The police stood there & made me call her there & then!
Google thier name with quotes around it, look on Facebook or another social media site, check a phone book, or ask around. It is illegal for websites to give out phone numbers unless that person provided it.