In this situation, the most appropriate step with the least invasive method would be:
Asking the caller if they would distance themselves from the knife and pills while you hear what has led them to this point.
This approach allows you to engage with the caller and potentially de-escalate the situation while prioritizing their immediate safety. After that, if necessary, you could involve emergency services or crisis intervention. However, the most critical first step is to ensure the caller's safety and begin a dialogue.