I know the conversation id and when i use the participant id as communication id for the customer participant, I get 404 error with resource not found.
The communicationId corresponds to the id of the call/session for the first participant (i.e. customer's participant for an inbound call).
If you invoke GET /api/v2/conversations/{conversationId}, this will correspond to the "id" of the call (call object in the "calls" array) in the customer's participant element.
If you invoke GET /api/v2/analytics/conversations/{conversationId}/details, this will correspond to the "sessionId" of the session (session object in "sessions" array, with "mediaType" = "voice") in the customer's participant element.