Samsung Galaxy S2 'Not Registered on Network'
From what I understand of SIMs, it's the SIM that is "not registered on network" since I'd guess you haven't kept up the payments on it when not using it, having moved to a new one.

If the phone works with your active SIM, it should work with a new activated SIM bought by the new owner, or an active SIM moved from the new owner's old phone.

Mind you, I have a phone with a useless SIM slot that's on a non-SIM network, so I might not have the right grasp of this, but that was what I had gathered about SIM phones - that for all intents and purposes your account was as portable as the SIM, and you could toss it into a phone and your account would follow; or get another SIM and have another account (ie, a country-local one when traveling.)