Why do calls go directly to Voicemail?

Is this all calls or select calls? There is a setting to send calls from numbers not in your contact list directly to voicemail. It is found in the Phone app (silence unknown callers), if you scroll down a bit.

Your phone may go directly to voicemail for a number of reasons, including:

• Airplane mode: If your phone is in Airplane mode, it won't receive calls. 
• Silent mode: If your phone is in Silent mode, all calls will go directly to voicemail. 
• Call blocking: If the number calling you is blocked, the call will go directly to voicemail. 
• Call forwarding: If call forwarding is turned on, calls will go directly to voicemail. 
• Network problems: If your service provider is experiencing network problems, calls may go directly to voicemail. 
• Ringless voicemail: If the company calling you uses a ringless voicemail system, calls will go directly to voicemail. 
• Bluetooth connections: If your Bluetooth connections are confused, calls may go directly to voicemail. 
• SIM card: If your SIM card has shifted, calls may go directly to voicemail. 

See this troubleshooting guide:

Why doesn’t my phone ring? - Apple Community

You should contact your carrier as the phone and voicemail are services of them.