Yes..you can put old sim card. I know only the paid unlocking service
I found an unlocking service at Ihav.net .
If you need an unlocking service , get it from this provider.Otherwise you may contact your network provider.
Hi free unlock code is not available for your sony ericsson k610i mobile. So better you can go for paid unlocking service. I would suggest this site Wickedunlock.com here you can get the unlock code and unlock your sony ericssion k610i mobile easily. To get the code you have to mention which country is locked now,network provider and imei number. Then they send unlock code through email.