I want the caller to leave a voice message if my phone is switched off or out of network coverage. How can I do it?

You can activate call divert to voice mailbox so that the caller can leave messages to your voice mailbox when you are not available.

To activate call divert, select Menu > Settings > Call > Call divert, (for dual SIM devices, select the SIM card to use), and select If not available or If out of reach > Activate > To voice mailbox and enter the voice mailbox number which you get from your service provider.