There are two ways:
First of all, through civil litigation
This behavior of taxi drivers belongs to unjust enrichment in civil law. Article 29 of China's General Principles of Civil Law clearly stipulates: "If unjust enrichment is obtained without legal basis and losses are caused to others, the unjust enrichment obtained shall be returned to the losing party". The so-called unjust enrichment means that others get benefits because they have no legal basis and suffer losses. Accordingly, a civil lawsuit can be brought to the court.
Second, through criminal proceedings.
According to the provisions of China's new criminal law, this behavior of taxi drivers has been suspected of constituting the crime of embezzlement. The crime of embezzlement refers to the illegal possession of other people's property, or the act of others forgetting the legally held and buried things and refusing to return them. Note: you must meet the conditions of large amount (depending on how much your sister's mobile phone is worth) and small amount. Even if you refuse to return it, you can't be punished for embezzlement.
However, in practice, it is generally believed that according to Article 79 of the General Principles of Civil Law, the lost property should be returned to the owner. Those who seize the lost property and refuse to return it will generally only be investigated for their civil liability.
You can go to the local police station for the record first and then solve it through the public security organs. If no settlement can be reached, the people's court can claim the rights.