You are only supposed to pass the tool schema inside this field, not the props named modal and tools. TRY THIS.

{
        "type": "voicemail",
        "function": {
          "name": "leave_voicemail",
          "description": "Leave a voicemail message when you detect you've reached a voicemail system"
        },
        "messages": [
          {
            "type": "request-start",
            "content": "Hi {{name}}, this is Linda at CDS. We work with IT firms and MSPs on secure backup, recovery, and cybersecurity. You can reach me at 531-223-1129. Again, this is Linda with Central Data Storage, at 531-223-1129. Looking forward to connecting."

          }
        ]
      }
    ]
  }