Hi Faisal,
Please check the following links that address this issue
https://supportforums.cisco.com/document/12370291/troubleshooting-fxo-disconnect-signal-issue
http://www.cisco.com/c/en/us/support/docs/voice/analog-signaling-e-m-did-fxs-fxo/8120-fxo-disconnect.html
Manish
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