Hi Ravi,
Here is some info including cable pinouts, for how these are setup;
The FXO expansion module supports a power failure port that connects directly to the central office (CO) in case of failure.To maintain an emergency connection during power outages, connect port 14(Expansion module 0) or port 24 (Expansion module 1) to the public switched telephone network (PSTN). Phones connected to port 8 or port 18 fail over to the PSTN connections through port 14 or port 24 during power outages.
From this good doc;
http://www.cisco.com/en/US/products/hw/modules/ps2797/products_installation_guide_chapter09186a008007c8ce.html#wp1022948
Q. What is FXO Power Failover?
A. The FXO Power Failure feature is a hardware feature built into the FXO cards that allows connectivity to an analog phone patched into the right pair of wires to be activated by a relay if power to the Cisco router containing the NM-HDA fails. This allows PSTN calls to be made via the FXO line normally connected to the router from a designated "red" phone in the office while power is out. For an FXO EM in slot 0 of the NM-HDA, the analog phone must be connected to wire-pair 14 (counting from 1 on the RJ-21 connector) to take advantage of the FXO Power Failover feature. For an FXO EM in slot 1 of the NM-HDA, the analog phone must be connected to wire-pair 24 (counting from 1 on the RJ-21 connector) to take advantage of the FXO Power Failover feature. If both EMs are populated with FXO modules, two "red" phones can be used.
From this Q&A doc;
http://www.cisco.com/en/US/products/hw/modules/ps2797/products_qanda_item0900aecd8017014a.shtml
Hope this helps!
Rob
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