Dial the number and let it ring until voicemail answers
Turn on Do Not Disturb and allow calls to go to voicemail
Silence the phone or enable Airplane Mode so the call is not answered
Forward calls to voicemail if your carrier or phone settings allow it
Use call-blocking or call-filtering features that send calls to voicemail
Set up a custom voicemail greeting and let the caller leave a message
If calling your own number, ignore the call until voicemail picks up