Calls on my iPhone go straight to voicemail

FIRST: Are the missed calls in Phone/Recents on the phone? If they are not the problem is in the network, because the calls aren’t getting to your phone. Contact your carrier: 

• check with your carrier that they don’t have a local outage, and that you didn’t have any settings that would have immediately invoked voicemail
• check up on them at Down Detector - outages

If they are in Recents here’s what to check:

• Check the MUTE switch on the left side of the phone, above the volume buttons. It should be towards the screen
• Check the volume setting. With the Home screen displayed, press the VOLUME UP button on the left side of the phone. It’s above the VOLUME DOWN button.
• Go to Settings/Do not Disturb. Turn it off if it is on. Also turn off Scheduled. With both off none of the other switches there matter.
• Go to Settings/Phone. There’s a switch for Silence Unknown Callers. If it is on any calls from numbers not in your contacts will go directly to voicemail.
• Also in Settings/Phone: Make sure Call Forwarding is off.
• Go to Settings/Sounds & Haptics. Make sure there is a sound assigned to Ringtone.
• Go to Settings/Screen Time/Communications Limits. Tap each of the 2 entries and make sure both are “Everyone"
• Restart your phone-->RESTART your iPhone, iPad, or iPod touch - Apple Support

Repeating myself:

If the old phone rings for the same number the number was never transferred to the new phone.

Do the missed calls appear in your Recents list in the Phone app? Tap Phone on the Home screen. Tap Recents.

If the old phone rings for the same number the number was never transferred to the new phone.

Do the missed calls appear in your Recents list in the Phone app?

Because the phone number is on the SIM card in the old phone, not in your Contacts. You need to either move the SIM card to the new phone or ask your carrier to transfer the number.

Remove or switch the SIM card in your iPhone or iPad - Apple Support

have you checked your settings?