What does "locked" (unlocked) adapter and "provisioning" mean?

What is an locked adapter (ATA / IP Phone)?
A locked adapter can not be used on any other network than the one it is locked to.  A locked devices cannot typically be unlocked (due to a custom firmware, that cannot be changed).  An unlocked adapter can be used with any provider supporting BYOD (bring your own device).
If purchasing your own adapter, you should ensure the device is unlocked (so that if can be used with any provider supporting BYOD (bring your own device).
An unlocked adapter may still be password protected, especially in the case where it is provisioned (see 'What is Provisioning', below).
What is Provisioning (provisioned)?
Provisioning is the means of automatically configuring an adapter (ATA, IP phone, sofphone, etc).  Provisioning may also be referred to as Managed.  Most providers provide provisioning on the adapters purchased from them. 
A provisioned adapter will be password protected.  Providers will not release the adapter password, on provisioned devices.  To access a provisioned adapter, contact your service provider to ensure provisioning is disabled, then perform a factory reset. Some providers may have a manual provisioning, that you activate, to configure the adapter (you will not typically need to contact the provider in this case).  Once the adapter has been reset, the default login details can be used to access the units configuration.
Why is my unlocked device password protected?
For security and privacy, unlocked devices will typically (and should) be password protected.  A provisioned adapter will be password protected (see 'What is Provisioning', above).
A provisioned device and an unlocked device are different.  A provisioned device will be password protected, but may still be unlocked (can be used on another network).
** Adapters purchased from VoIP Much are unlocked, provisioned (by default) and password protected. To access the adapter, you will be required to factory reset the unit.  Our provisioning is manually activated, so there is no risk of the unit downloading our settings again automatically after the reset.