You, and the person running the service (Eric Dormer). We do not sell data, and we do not share your extension with anyone except the people you give it to. Members are listed in the public directory only if they ask.
Voicemail and messages are deleted when you close your line, and in any case after 90 days. Call records are kept for 12 months to detect abuse of the shared number.
Voicemail and text are stored on servers in Canada and the United States. Treat them as private but not secret: do not leave banking details in a voicemail.
Email admin@xfast.top and say "delete my line". We will remove the record and the messages.