the "sessionId" attribute (session with mediaType = voice in the sessions array) of a customer's participant (purpose = customer)
i.e. in the result of an analytics conversation details request with GET /api/v2/analytics/conversations/{conversationId}/details
the "id" attribute (in the calls array - the calls array which is at the participant structure/object level) of a customer's participant (purpose = customer)
i.e. in the result of a conversation request with GET /api/v2/conversations/{conversationId}
As you can notice, the "sessionId"s from the Analytics request (customer's participant - sessions array) correspond to the "id"s from the Conversation request (customer's participant - messages array). Same values.