Samsung Galaxy S2 'Not Registered on Network' I have a Samsung Galaxy S2 that I want to sell.  I have reset it to factory settings.  However, I have a problem.  When I bought the phone new, it was on O2.  I changed fairly recently to Virgin Mobile.  Now, however, I have taken the SIM out of it to put it in my new phone.  The original SIM (O2) was PAYG, which changed to SIM only contract and that was still the case when I changed to Virgin.  Now, when I put the old (O2) SIM back into the phone, it says Not Registered on Network.  I've tried a factory reset and messing with the APN settings but nothing seems to work.  I'm at my whit's end. Please help.
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.)