1. Try to contact the finder: If you know the finder's contact information, you can try to communicate with him and request to return the mobile phone. Stay calm and polite, and explain the importance of mobile phones.
2. Call the police: If the problem cannot be solved through communication, you can call the police. Provide detailed information and evidence to the police, such as the model and color of the mobile phone and the description of the finder. The police will investigate and try their best to help find the mobile phone.
3. Seek legal help: If the police can't get their mobile phone back or need further legal support, they can seek the help of a lawyer. Lawyers can provide professional legal advice and help to take appropriate legal action.