404 on remove participant endpoint for conference

Getting a 404 when calling POST /api/v2/conversations/voice/{conversationId}/participants/{participantId}/remove. The participant is definitely in the conference, but the API says not found. I tried passing the conversationId directly. Should I be using the conferenceId instead? Here is the curl request:

curl -X POST "https://api.mypurecloud.com/api/v2/conversations/voice/abc-123/participants/def-456/remove" \
 -H "Authorization: Bearer $TOKEN"

Response:

{"code":"not_found","message":"Conversation or participant not found"}