Voluntary is the sense of someone(s) in a position of authority or power asking if you're willing to submit to a voluntary procedure.
I've be interviewed by police twice after being charged with crimes that carry potential prison sentences. The interviews are voluntary in the sense you're not required to answer any of the questions, and I could have chosen to wait in a cell for my lawyer to turn up days later ... but I volunteered to respond to each question by making a mockery of their questions and processes.
These people in authority, they are not there to help you, with their badges and their guns and their attitude problems, and I don't believe it's possible for someone to "volunteer" when such a power imbalance exists.
It's an absurd use of the word. You can't "volunteer" when the other person is carrying a gun.
Absolutely you could have waited for a lawyer. You voluntarily decided not to.
"voluntary" - done, given, or acting of one's own free will. You said yourself that you could have waited, but decided not to.
Alternatively you could have just done a "no comment" interview, which - let's be honest - is what the lawyer would have advised you to do, no matter if you're guilty or not.
