I am frequently bothered by robocalls to my mobile phone.
Many of my clients have this problem as well
The following is something I have implemented that is helping a lot.
I create a new DID with our SIP Trunk provider.
I create a 3CX inbound rule to forward calls to this DID to a new Digital Receptionist.
The DR greeting just says "Press 1 to leave a message for Allen" and then option 1 forwards caller to VM.
I find that most robocallers do not know how to respond to the "press 1" request.
In the US most mobile carriers have a programming option called something like "forward busy/no answer".
This effectively allows you to replace an external option (like 3CX) for the mobile carriers voice mail.
Verizon uses *71 followed by 10 digit phone number to be transferred to.
I program forwarding of the call to the DID mentioned above.
Now, for incoming calls on my mobile if I see a Caller ID that I don’t recognize I don’t answer or reject and let the call go the the 3CX DR.
If it turns out to be a human caller they can press 1 and leave a message which I can return.
Most of these calls turn out to be robocallers and they don't make it past the DR.