for voice calls, the best thing I ever did was set up my own asterisk system with a DID that answers as an IVR saying something like "you have reached $NAME, this is an anti-spam system, please confirm you are a human by entering the digits five three zero nine to be connected".

If people then put in the DTMF tones for 5309, or whatever code it might be this month, it initiates an outbound call to my actual cellphone and bridges the calls together.

In many cases I don't give out my direct cellphone number to 3rd parties, or anyone who isn't a close family member or colleague, I only give out the DID of the system with the filtering IVR on it.

I do still receive the occasional coldcall spam on my cellphone's direct number, which isn't entirely avoidable, but those are only random calls which aren't looking for me in particular. By only giving out the phone number of the filtering DID, I avoid my number being entered into many random third parties' CRM systems which are inevitably subject to data leaks, copying, data sharing with other 3rd parties, sales and marketing campaigns, etc.