My approach is simple. If it comes from a trusted referal, then I use my free Clarity VIP link and still conduct the call using Clarity. If it comes in cold, and there's no association to a cause, alumni or area of interest, then I just say "I'd love to talk, but I'm heads down with Clarity and other existing commitments - if it's truly urgent and requires my advice, I conduct all these request via Clarity here: http://clarity.fm/danmartell"

That works well and sets the expectations.

Clay Hebert http://clarity.fm/clayhebert wrote a great post about that here:
https://medium.com/@clayhebert/the-new-way-to-say-no-6a7d29282c1d