Answer:
Incoming calls may go straight to voicemail if your phone is on Do Not Disturb mode or has the Silence unknown callers feature enabled. Poor network signal or coverage can also cause incoming calls to go to voicemail. Call forwarding settings may also be the issue.
On an iPhone, calls may go straight to voicemail if the phone has no service, Do Not Disturb is turned on, or a Carrier Settings update is available 1. To fix the issue, you can disable Do Not Disturb and call forwarding, check your network coverage, or contact your mobile provider.