What do you mean when you say "decode" any mobile phone? I've never heard of "decoding" a mobile phone. I assume you mean unlocking a phone. And the process can vary from phone to phone.
For example, with Android powered phones, it usually involves forcing a fake update to the firmware that gives the you Superuser privileges (similar to Administrator privileges in Windows). This process is called "rooting".
If you want to unlock a phone, you're best bet is to search on Google. Keep in mind that unlocking a phone voids the warranty.