I have an iPhone 16. Call Blocking & Identification is turned off. Announce calls is Always. Do Not Disturb is off. Silence Unknown Callers is off. Vapi phone number is added to my contacts. I call my personal iphone using the Vapi number, nothing happens, 1 minute later or so i get a notification that a new voicemail exists. What could be going on? Does Apple detect that Vapi phone numbers are spam and auto blocks them?

You need to turnf off ios default feature of voicemail it's from ios not from vapi.