The number +251857 is the official Ethio Telecom Voicemail system number in Ethiopia.
this number under "Forward when unreachable." This is standard carrier configuration. It means that if your phone is completely switched off, out of battery, or has no network coverage, any incoming calls will automatically divert to Ethio Telecom's network voicemail so people can leave a message.
Your phone remains completely secure because "Always forward" is set to "No number set." Your calls are only routing to the carrier system when you are entirely offline