I'm thinking about buying VOIP adapter for home use. I already use SIP software on my PC and Mobile, but i think it would be easier to just pick up the phone at any time and call. And I could recycle old PSTN phone for that. The only thing I noticed is if I look for them on eBay, most of them are "unlocked" (like this one: UNLOCKED Linksys SPA1001 one FXS VOIP PHONE ADAPTER for Asterisk Elastix Trixbox).
I do not understand why. Isn't it that you just put your SIP provider proxy and password and it just should work? Why they were locked in the first time. When I google that, i find many sites that talk about unlocking them, but I'm not sure why they were locked in the first place.