Hi Alireza,
Here is the sequence to reset the 7970 to factory defaults;
Factory Reset
A factory reset of the Cisco IP Phone restores the phone is reset to its factory state. This procedure reloads the default firmware image, deletes call histories from the call logs, and resets all configuration settings to their factory values.
To perform a factory reset of a phone, perform the following steps. The phone must be on a network that supports DHCP.
Procedure
Step 1 Unplug the power cable from the phone and then plug it back in.
The phone begins its power up cycle.
Step 2 While the phone is powering up, and before the Speaker button flashes on and off, press #.
Each line button flashes on and off in sequence.
Step 3 Press 123456789*0#.
You can press a key twice in a row, but if you press the keys out of sequence, the factory reset will not take place.
After you press these keys, the phone will go through the factory reset process.
From this 7970 doc;
http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a00801d66cb.html#wp1087544
Hope this helps!
Rob
