Dear Vijayaraj,
As i know, this alarm may arise when the PCM synchronization for the site is not OK (bad quality Transmission..). The loop test is between the BTS and the transcoder (TRAU) which needs that the BTS is well synchronized with the TRAU.
By the way, Vijayaraj what is you working specialty in GSM (BSS,NSS), do you have Ericsson experience?
I hope this helps..
Regards.