Be very careful not to imply that anyone can become a volunteer simply by calling up or raising a hand. It's guaranteed that you'll end up with a screening problem! Or with someone who says, "what do you mean I can't do this?" The best thing is to say some variation of: "now accepting applications," "call for an interview," or "let's talk about whether this is the right assignment for you." Then you have established your intention to find the best candidates, which may mean turning some applicants away. Hint: Even if you take the first person who applies, it's much more motivating to say "We're delighted that you qualify for this volunteer role" than to say "Since you're breathing, you can do it"!
Susan J. Ellis