If your phone does NOT have a SIM card slot, then it cannot be unlocked. Please note that the SIM card slot can be found on the back of the phone behind the battery. The image below shows what a SIM card slot looks like.
Phones that have a SIM card slot generally come from GSM network carriers who use SIM cards as a primary means of communication. This excludes network carriers like Boost Mobile who operates on the iDen network.
There are also CDMA world phones from network carriers such as Verizon and Sprint which carry both an IMEI (Serial number) and MEID number (Unique number used to identify CDMA phones). If you are able to locate the IMEI on the back of the phone behind the battery which is a 15 digit number, then your phone can be unlocked. If no IMEI exists, then the phone cannot be unlocked.
All IMEIs can be found on GSM phones by dialing *#06# on your dial pad. To find the IMEI on a world phone such as the Verizon Motorola Droid 3, you can find the 15 digit IMEI on the back of the phone underneath the battery. On CDMA world BlackBerries such as the BlackBerry Bold 9650, or the Bold 9930, you can find the IMEI by going to Options => Status or Options => Device => Device and Status Information.
Almost all IMEIs begin with 35xx, 98xx, 01xx, 0044xx.