How do I get the voicemail transcription using java SDK?

Provided that you are using the RingCentral Java SDK, you can read the transcript file as shown below.

public void read_message_store_voicemail() throws RestException, IOException{
        ListMessagesParameters parameters = new ListMessagesParameters();
        parameters.messageType = new String[] {"VoiceMail"};
        
        var response = restClient.restapi().account().extension().messagestore().list(parameters);
        for (GetMessageInfoResponse record : response.records)
        {
            if (record.attachments != null)
            {
                for (var attachment : record.attachments)
                {
                    var fileName = "./src/test/resources/" + record.attachments[0].id + "_voicemail.txt";
                    if (attachment.type.equals("AudioTranscription"))
                   {
                    var res = restClient.restapi().account().extension().messagestore(record.id).content(attachment.id).get();
                    Path path = Paths.get(fileName);
                    Files.write(path, res);
                    }
                }
            }
        }
    }

Hope this helps.