She works there on a voluntary basis.

她自愿在那里无偿工作。

《牛津词典》

We run the meals-on-wheels service on a voluntary basis.

我们基于自愿的原则，开展上门送餐的服务。

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And those with the title of "National Model worker" or certificates of "senior workers" or "senior technician" are expected to get Beijing's Hukou on a voluntary basis.

这些外地农民工如获得全国劳模称号，或取得高级工、高级技师职称的，根据本人意愿，有望在北京落户。

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