The Buddy System is a central safety routine in sex work: A trusted person (colleague, family, advice center) knows the date, time, location, customer name before each appointment and reports if the sex worker does not send a sign of life after the agreed time.
Tools for this: WhatsApp location sharing, Find My Friends, dedicated apps such as "bSafe" or "Watch Over Me". Advice centers offer an anonymous buddy function for sex workers without a private network.