code: public DtmfResponse sendDtmf(String uuid, String digits) throws IOException, NexmoClientException {
          return dtmf.put(uuid, digits);
      }
docs: Send DTMF codes to an ongoing call.
  
  @param uuid   (required) The UUID of the call, obtained from the object returned by {@link #createCall(Call)}.
  This value can be obtained with {@link CallEvent#getUuid()}
  @param digits (required) A string specifying the digits to be sent to the call. Valid characters are the digits
  {@code 1-9</tt>, <tt>#</tt>, <tt>*</tt>, with the special character <tt>p} indicating a short
  pause between tones.
  
  @return A CallInfo object, representing the response from the Nexmo Voice API.
  
  @throws IOException          if a network error occurred contacting the Nexmo Voice API.
  @throws NexmoClientException if there was a problem with the Nexmo request or response objects.