Loop Break Test is not only useful in ISUP.Normally the PCM itself has some maintanence activty by sending some specific pattern of Bits in SYNC TS-0.By Physically looping…that is Giving the TX out Put to RX Input these BIT Patters returned to the same PCM Maintainer-ET.Which anakyzes the received pattern and says the PCM is WO.If there is any break it wont receive this BITS.So it will say the PCM is NOT OK.
With LOOP BREK You can locate the break in the PCM Link.Local Loop & Remote Loops and Intermediate Loops are possible to test.
Rgds