If the port is really faulty, then you cannot repair it as you would need to send it back to Quintum for repair. However, many times a failure to detect dialtone may be a problem with the cabling or line. If your unit has the FXS/PBX ports active, you can try connecting the FXO/PSTN port that is failing to a FXS/PBX port directly and run the test again to see if it detects dial tone. If it does then the port is ok.
Also, if you are using GSM or Fixed Wireless Terminal for connection, try a landline to that port as a test.