Turn off Live Voicemail.
Open Settings → Apps → Phone → Live Voicemail and switch it off. On older iOS versions, Phone may appear directly in Settings. Live Voicemail can answer on the iPhone itself.
Visual Voicemail is different: it displays carrier voicemail messages. If your iPhone offers no off switch, ask your carrier whether its Visual Voicemail feature needs disabling for external forwarding. Do not cancel the voicemail mailbox or remove forwarding features without the carrier’s guidance.
Apple voicemail guide ↗
