Spam calls

I have been receiving a very large influx of spam calls on my phone. I continue to block the numbers however they keep calling from a new phone number. I also hate when I decline the call, it lets them leave a voicemail, as many spam calls are now doing that too.

I put my phone on do not disturb, however, once again, they leave a voicemail - even the robocall calls leave an automated a voicemail as well.

I am with Public Mobile - and they do not have an apps/software for this. I would like the option to reject certain calls and not have them bounce to my voicemail. Any further options for your iPhones? I have looked into some apps, but many reviews are making me feel they may not work well enough to bother.