Unfortunately, I don't think that would be technically possible. As mentioned in my previous post, blocking or rejecting calls on your phone only prevent the calls from ringing your phone. It does not block the calls on the network. From the network perspective, that would appear the same as someone missing a call. The network cannot know whether an unanswered call was rejected or simply missed.
Hope this helps 😀
Welcome to the community!
Unfortunately, until the technology to unmask the true number of spammers/scammers is readily available (not simply the number shown on the caller ID), the only solution currently is to block the numbers on your phone. However, you should note that blocking the calls only prevents the calls from ringing your phone. Since the number block is on your phone, the calls would have had to already have reached your phone. It would be similar to a missed call. As you are aware, it does not prevent the callers from leaving voicemail. There are (at least were) some apps which claimed to also prevent callers from leaving voicemail. They worked by answering and almost simultaneously ending the calls. While those apps do prevent calls from going to voicemail, the calls are also considered 'answered' and therefore count as airtime.
The only other way to prevent callers from leaving voicemail would be to disable your voicemail or have the numbers blocked at the network level. Unfortunately, blocking phone numbers at the network level is not feasible on a per customer basis. The mobile providers have implemented Universal Call Blocking which is meant to block calls from malformed phone numbers. That said, I'm doubtful whether it will have much of an effect on the amount of SPAM or SCAM calls since many of them appear to have properly formed phone numbers. In addition, a new technology, STIR/SHAKEN (Secure Telephone Identity Revisited/Signature-based Handling of Asserted Information Using Tokens) is being adopted to further reduce the amount of nuisance calls (see here). However, I have my doubts as to whether those measures will reduce the amount of SPAM calls. My understanding is the STIR/SHAKEN will only identify possible SPAM calls and mark the calls as suspected SPAM.
Hope this helps 😀