I don't quite understand what you mean here by the state being involuntary and anarchism being voluntary. If you point is that the minority would walk away somewhere if they disagree with the majority, I don't think this relocation will be by their free will and accord. Actually, they will be coerced to do so by the majority. It happens daily in life even without state and its laws. But exactly the introduction of state in the form of the law allows to break this otherwise circular argument...
You are describing panarchism not anarchism. Also most anarchists are pro law, we imagine a society that has plenty of laws. We just imagine that those laws are provided in a competitive marketplace so that they can improve with time for the same reason that microprocessors get faster every year and cars get better gas mileage every year. So if you dont like the laws that you are presently governed by than you dont have to move away, you just change service providers.
I just paraphrase what others are saying here. But what you say has also been described somewhere earlier in the thread. What if I just don't want to pick up any of the "law providers"? If I don't choose any, would I be outlawed? If not, then it simply doesn't make any sense to choose any "law provider"...