Phone calls sent to voicemail

• Check your DND status - Settings > Do Not Disturb; Use Do Not Disturb on your iPhone, iPad, and iPod touch - Apple ...
• Check BedTime Status - Clock App; Use Bedtime to track your sleep on your iPhone - Apple Support
• Check your Sound Volume; Adjust the volume on iPhone - Apple Support
• Check your Ringer switch; If you hear no sound or distorted sound from your iPhone, iPad, or ...
• Check Call Audio Routing; Call Audio Routing Bluetooth / Speaker / Automatic
• Check Auto Answer Calls; Turn off Auto Answer Calls
• Check Connection with Bluetooth Audio; Stream audio from iPhone to your hearing devices - Apple Support
• Check whether in Headset Mode; If your iPhone is stuck in headphone mode - Apple Support

Some of the reasons are listed below which may direct your calls to VoiceMail

1. DND status
2. Bed Time schedule
3. Call Audio Routing method
4. Ringer Volume level
5. Call Forwarding to another number
6. Silence Unknown Callers [Enable] Calls status : Only numbers in Contacts, Mail, and Messages ring your phone. All others are diverted to VM
7. Have you done any of these; decline a call and send it directly to voicemail, by doing one of the following:
   1. If iPhone is unlocked, tap Decline.
   2. If iPhone is locked, press the side button or Sleep/Wake button (depending on your model) twice quickly.
   3. On your EarPods, press and hold the center button until you hear two low beeps.
   4. To be reminded to return the call, tap Remind Me, then choose when you want to be reminded.
   5. To respond with a text message instead of answering, tap Message, then choose a reply or tap Custom.
   6. To create your own default replies, go to Settings > Phone > Respond with Text, then tap any default message and replace it with your own text.
   7. Block unwanted callers: In Favorites, Recents, or Voicemail, tap  to see a caller’s contact card, then tap Block this Caller. Or go to Settings > Phone > Blocked Contacts. You don’t receive voice calls, FaceTime calls, or text messages from blocked callers. All are diverted to Voicemail