1.- FIRST STEP:
	1.- Go to include/class.thread.php.

	2.- Search for:
			// XXX: Consider collaborator role
			elseif ($mailinfo['staffId']
					|| ($mailinfo['staffId'] = Staff::getIdByEmail($mailinfo['email']))) {
				$vars['staffId'] = $mailinfo['staffId'];
				$poster = Staff::lookup($mailinfo['staffId']);
				$vars['note'] = $body;
				return $ticket->postNote($vars, $errors, $poster);
			}

	3.- Comment this line:

				return $ticket->postNote($vars, $errors, $poster);

	4.- Add this line below:

				return $ticket->postResponse($vars, $errors, $poster); // LINES CHANGE

				
	5.- The code should now look like this
			// XXX: Consider collaborator role
			elseif ($mailinfo['staffId']
					|| ($mailinfo['staffId'] = Staff::getIdByEmail($mailinfo['email']))) {
				$vars['staffId'] = $mailinfo['staffId'];
				$poster = Staff::lookup($mailinfo['staffId']);
				$vars['note'] = $body;
				//return $ticket->postNote($vars, $errors, $poster);
				return $ticket->postResponse($vars, $errors, $poster); // LINES CHANGE
        }


2.- SECOND STEP:
	1.- Go to include/class.ticket.php.

	2.- Add this function after the last function ("checkOverdue()") closure:

	// postResponse is a copy of postNote with some changes to make it post a staff response instead of a note.
	function postResponse($vars, &$errors, $poster, $alert=true) {
        global $cfg, $thisstaff;

        if(!($response=$this->getThread()->addResponse($vars, $errors)))
            return null;

        //Set state: Error on state change not critical!
        if(isset($vars['state']) && $vars['state']) {
            if($this->setState($vars['state']))
                $this->reload();
        }
			
        $dept = $this->getDept();

        if($thisstaff && $vars['signature']=='mine')
            $signature=$thisstaff->getSignature();
        elseif($vars['signature']=='dept' && $dept && $dept->isPublic())
            $signature=$dept->getSignature();
        else
            $signature='';

        $variables = array(
                'response' => $response,
                'signature' => $signature,
                'staff' => $thisstaff,
                'poster' => $thisstaff);
				
        $options = array(
                'inreplyto' => $response->getEmailMessageId(),
                'references' => $response->getEmailReferences());

        if(($email=$dept->getEmail())
                && ($tpl = $dept->getTemplate())
                && ($msg=$tpl->getReplyMsgTemplate())) {

            $msg = $this->replaceVars($msg->asArray(),
                    $variables + array('recipient' => $this->getOwner()));

            if($cfg->stripQuotedReply() && ($tag=$cfg->getReplySeparator()))
                $msg['body'] = "<p style=\"display:none\">$tag<p>".$msg['body'];
            $attachments = $cfg->emailAttachments()?$response->getAttachments():array();
            $email->send($this->getEmail(), $msg['subj'], $msg['body'], $attachments,
                $options);
        }

        if($vars['emailcollab'])
            $this->notifyCollaborators($response);

        return $response;
    }