the default phone lock code for sony ericsson appears to be: 0000
if the phone isn't taking that, and you don't remember changing it then there is little chance your cell phone provider will be able to tell you anything different.
the phone lock code protects the phone, obviously

the PIN code protects the sim (if you forget your PIN you provider can help you by giving you a PUK code)
Worst case scenario: you can reset the phone lock code using a 2in1 multi-server USB cable and a total multi-server credit. you'll pay around $30 for those, or you can send your phone away to have it reset. or you can do a search on local classified websites like: craigslist.org i'm sure there is someone advertising cell phone unlocking/flashing services and they should be able to get that reset for you.