Find all files (-type f) and execute a sed command (-exec sed) to replace any numbers in the format of three digits followed by a hyphen and four digits (e.g. 123-4567) with ten asterisks (**********) for each file found ( {} \;).
find . -type f -exec sed -i '' 's/[0-9]\{3\}-[0-9]\{4\}/**********/g' {} \;