Typically, if it looks like an odd number, I let it go to voicemail. My rule used to be that if it were an out-of-state number, it went to voicemail...that is now out the door with cell phones and portability of numbers. I still answer most calls. If they leave a voicemail, I figure it "might" be legitimate but most of the time they do not bother. I agree, prospective clients that are LEGITIMATELY interested in your services, and are not just fee shopping, will leave a voicemail and be accepting of having to wait a reasonable amount of time for a callback. Also, most of my new/prospective clients e-mail me.
I block numbers automatically since I answer most of my business calls via the RingCentral apps on my PC or cell phone, and it is a tap of a button to do so. I could also screen calls, but I feel like that would be offensive if they suddenly find they are going to voicemail instead of having their call answered and they're a prospective client.
Still, I do try to answer most calls even if a robocaller or advertising. CPA firms have too poor of a reputation for not answering or not being accessible, and I have always received high praise for my accessibility and responsiveness. Hence, immediately blocking undesired numbers. Also, in cases where my services involve working with clients of my clients (owners within an HOA/POA being prime example), I add them to my contacts so I can recognize their numbers/names.