Hello, thank you for your query.
The police have a right to stop and question a person at any time. However, in most cases, the person does not actually have to stop or answer any questions. If the police have no reason to think a person has committed a crime, the refusal can't be used as a reason to arrest.
Whether the police have 'reasonable grounds' to suspect anyone involved in a crime, they are not legally allowed to look through one's phone unless given permission or they have obtained necessary legal documents.
If your phone is password protected, you are not required to disclose it. However, problems could arise if a person is being investigated for terrorist activity or s*xual offenses against children.
Hope this solves your query.