I believe it's definitely the customer/screening system on customer's end, ending the call as indicated by the call end reason at the bottom of your screenshot. There are a bunch of call end reasons specified by Vapi and are pretty helpful in narrowing down what exactly is going on in the backend. People's default behaviour when dealing with unknown numbers is to let it go to VM or hang-up. So, if people are requesting the calls, I think they might not associate that number with you. What voicemail detection provider were you using before?