machan, I hope Shana Khan's method will work, right on the first time as, I have heard that if that doesn't work first time, then the mobile can get stuck forever. I am telling you that, cuz I too once had the same unlocking problem in SL, with a foreign used phone by me.
My recommendation for you is, to take it to a authorized unlocking agent.
These guys seems to be doing so but I have not tried. But they seems to be running a website, so maybe it's a good company.
The Gadget Store (Pvt) Ltd Showroom
No 279E R.A.De Mel Mawatha
Colombo 03.
Sri Lanka.
http://www.techgsmsl.com/Phone%20Repair.htm
Cheers!