IteadStudio Gboard (GSM Arduino board). DTMF does not work correctly

I bought a GBoard from IteadStudio (http://imall.iteadstudio.com/special-product/im120411004.html). The GSM functioncallity is working fine, but today I tried send and receive DTMF signal, but without success.

For some reason the class commented (disabled) in GSM_Shield.cpp library, line 765 is one sample.
Library:ftp://imall.iteadstudio.com/IM120411004_GBoard/Documents/Lib_IM120411004_GBoard_SIM900.zip

See an excerpt:

/**********************************************************
Checks if DTMF signal is valid

return: >= 0x10hex(16dec) - DTMF signal is NOT valid
        0..15 - valid DTMF signal
**********************************************************/
/*byte GSM::GetDTMFSignal(void) //LINE 765
{
  static byte last_state = LOW; // initialization
  byte ret_val = 0xff;          // default - not valid

  if (digitalRead(DTMF_DATA_VALID)==HIGH
      && last_state == LOW) {
    // valid DTMF signal => decode it
    // ------------------------------
    last_state = HIGH; // remember last state

    ret_val = 0;
    if (digitalRead(DTMF_DATA0)) ret_val |= 0x01;
    if (digitalRead(DTMF_DATA1)) ret_val |= 0x02;
    if (digitalRead(DTMF_DATA2)) ret_val |= 0x04;
    if (digitalRead(DTMF_DATA3)) ret_val |= 0x08;

    // confirm DTMF signal by the tone 5
    // ---------------------------------
    SendDTMFSignal(5);
  }
  else if (digitalRead(DTMF_DATA_VALID) == LOW) {
    // pulse is not valid => enable to check DTMF signal again
    // -------------------------------------------------------
    last_state = LOW;
  }
  return (ret_val);
}
*/

Is there any reason to come to the library this way,.
Considering this, someone could help me with the following functions:
1-EnableDTMF
2-GetDTMFSignal ()
3-SendDTMFSignal ()

Thank you very much for any hints.