VOIP Adaptor locked vs unlocked

"Locked" means this ATA can only be used with one particular VOIP service provider such as Vonage. This is similar to cell phones locked to certain phone service providers.

"Unlocked" ATA means it can be used with any VOIP service providers. Certainly some configuration is still required per VOIP service provider instruction.

Usually the seller will disclose ATA is locked to certain VOIP service providers or, it's unlocked. If it's not part of description, you can always to check with the seller to be sure.