My friend has Samsung SGH-N710 set. He want to lock his phone with password but he doesn't know the default password for it. What is the default password for phone? Is there any other way to set a password in that set?

To restrict outgoing calls u need to activate the Outgoing Call Barring facility,provided its offered by ur service provider.For Airtel the default Barring password is 1234.

^^
I have tried that but it says, wrong password.

@ Tech Geek
Thx friend. I asked my friend to try that & it worked.
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One more thing, now my friend has changed its password but he doesn't know that, how to lock it? Moreover he want that, no one can make any phone calls in his absence.

it is 8 zeroes

The default password is usually 12345 or 00000 in mobile phones. I think Samsung prefers the second one.