When I receive a call from a phone number I don’t recognize, I almost always send the call to voicemail. I have found that most of these phone calls are either marketers trying to sell me something (like SEO people) or they are inquiries about a domain name.
I think it’s a wise move to send inquiry phone calls to voicemail. In fact, it might be smart to set up a phone line that is only for your Whois records so that all calls can be automatically sent to voicemail without your having to wonder if it’s a friend calling from a different phone number or someone else calling about personal matters.
Sending inquiry phone calls to voicemail is good for a number of reason. Obviously, the most important reason is that you’ll have time to research the potential buyer or company making the inquiry. The person on the other end of the line is likely prepared to discuss your domain name, and you might be at a negotiating disadvantage if you pick up and get right into a negotiation without any preparation.
When you receive an inquiry over the phone, it might not be an opportune time for you to speak. I hate chatting when I am busy with other things, and if I want to have a conversation about a domain name I own, it should be at a time that is convenient for me. I can call the person back when I am ready.
Finally, assuming you don’t have the price of your domain name in front of you, you’ll want to do a bit of research on the domain name and market for similar domain names before you price yours. Yeah, you might have paid $5,000 for a domain name and will be happy with $10,000, but if you find that a buyer has been paying up to $30,000 for particular domain names, it will benefit you to have that information.