My dad’s iPhone 13 Pro Max goes to voicemail sometimes

Is the "sometimes" always when the call is from someone not in his Contacts list?

If Settings > Phone > Silence Unknown Callers is turned on, calls from those not in Contacts go directly to voicemail. See the information in: Detect and block spam phone calls - Apple Support including the following:

With iOS 13 and later, you can turn on Silence Unknown Callers to avoid getting calls from people you don’t know. This blocks phone numbers that you've never been in contact with and don’t have saved in your contacts list. If you’ve previously texted with someone using their phone number or if a person has shared their phone number with you in an email, a phone call from that number will go through.