This guide will explain how to check if your Sony-Ericsson is locked or unlocked and what that means.

Locked -

When a GSM handset, one that requires a SIM chip, is locked it is restricted in how it can be used. There are a variety of locks that may be in place. Most frequently seen is the Network Lock. This is what prevents the handset from being used with a carrier other than AT&T. Throughout a Z310a locked to AT&T will be used as an example. This applies to most Sony-Ericsson handsets.

This is an example of the
SmartChip Lock screen on
a locked Z310a.

Unlocked-

An unlocked GSM handset is one where the restrictions on use have been removed. This allows the use of any active SIM in the handset. AT&T, SunCom, Unicel, I-Wireless, T-Mobile, Vodafone will all function in an unlocked handset if the handset is in their service area and has an active SIM for the service.

This is an example of the
SmartChip Lock screen
showing all locks open on
a Z310a.

So, How to Check?

Checking to see if your handset is unlocked is quite simple.

Turn on the handset
When it finished booting enter >*<<*<*, ignoring everything on the screen until finished. The carets are left and right on the circular navigation key above the keypad. The * is the * key at the lower left of the keypad.
You will then see the Service Menu.
Using the left soft key Select "Service Info"

This brings up the Service Info screen.
Scroll to "SmartChip Lock" and select it using the left soft key.

If your locks look like this then you are safe to say that your Z310a is an unlocked handset. Note that all five padlocks are open.
If your locks look like this, even a single padlock closed, your handset is locked. It is that simple. In this instance it is locked to a network.