Is volunteer lab work a good practice? I met a  professor last year through an online search and expressed my interests to his research and his advanced lab technique, in which I lack of experience. 

I also asked for a PhD opportunity from him, he says that he has no money or limited funding now, so I agreed to work for 3 months for free. Would be practical that this volunteer experience could lead to a PhD after this 3 months? On one side I am not happy with working for free, as I had employment for 1 year as a research assistant. On the other side, this potential professor appears nice to employees around and keeps his words, and has good tracks of papers.

But am I just wasting of time, what actions should I take to figure this out before too late?

Supplementary information: He said that he was on travel, so I was off for a while. He did not contact me till I found him. Is this normal or just my altitude should be changed? I feel not well, maybe he does not really want me as a fellow at all? 

I am really frustrated, should I go further or not, is he just being polite to accept me to practice the techniques in his lab for 3 months? Am I wasting of time to even practice these test methods?
If you WANT to volunteer in the lab as a learning opportunity, it can be a great practice.

If you don't want to, decline the offer and either look elsewhere or come back when his budget can handle bringing someone else in... and hope that he hasn't already found someone who was pleased to work as a volunteer and delighted to be paid.

The world is what it is, not what we wish it should be.