If you find that your phone is going straight to voicemail even when it's on, there are a few potential explanations.
One possibility is that you have Do Not Disturb mode enabled on your phone. This feature silences all incoming calls and notifications, so if you're expecting a call, be sure to check and see if Do Not Disturb is turned off.
Another possibility is that the person calling you has their number blocked on your phone. If this is the case, their call will go straight to voicemail without ringing through to your phone at all. To check if a number is blocked, go to your Phone app and select the Recents tab. If you see a blocked call in your recent calls list, it means that the caller has been blocked.
Finally, it's also possible that there's an issue with your carrier or network connection. If you're having trouble receiving calls, try restarting your phone and checking for any updates for your carrier or network provider.