Phone lock code ? Did you mean carrier lock ? Make sure that your Sony Ericsson accept other sim cards or ask you to enter network unlock code ? If it asks code, you need to give unlock code to unlock your Sony Ericsson p990i from network lock . You can get it from any of the online vendors like ProUnlocking.com to unlock it .
OR
If you want to unlock your phone from security code lock, use default codes to break it .