Locked vs unlocked old phones

Yes you can sell or donate them or whatever else you want with them. You own them. Locked simply means they can only be used on ATT’s network. Depending on how old they are, that might not even be true anymore either. 

unless you requested unlock codes they are locked to at&t, some older phones have had their IMEI purged and cannot be unlocked.phones that the manufacturer no longer support cant be unlocked.

Yes, remove any old SIM cards whether you sell them or donate them. They likely don’t have any usable data but just to be safe, remove them. They can’t  be re-used, anyway.

Please factory reset, removing ALL data.   

Unless factory reset, data CAN be recovered off the phones. 

Thank you for all your help! I will just consider them locked to ATT then. I'm sure these phones are pretty old, at least 3 years or more. Thank you again! 

I'm sorry but I thought I was ok but now I have another question - should I remove the SIM card? I mean, someone else would have to use their own right? I deleted everything from the phone but the photos. Should I just remove that little card and sell or donate as locked? 
Thank you all for your help.

Thank you lizdance40 and sandblaster!  I was not able to do factory reset on some of them but I did remove the SIM card and will just call these "att possibly locked" lol.

Thanks again!  

As written, unless you unlocked them, they are locked to AT&T.  

I strongly discourage giving away a phone that isn't factory reset.  Data is not deleted, even if you deleted it, unless the phone is factory wiped, and can be recovered. 

When I tried to factory reset, it kept asking me for a security code. These are old phones, even if they had codes I have no idea where they were or what they were!  These are like flip phones and the slide out keyboard. I think they were just used for calls, no credit card stuff so I think it will be okay.

They are looking for your screen lock code.  That means it's user locked.  Which only the person who put the lock on can remove. 

If you don't know it, no one can use that phone.   It's garbage.  Except to a skilled hacker. To them it's a gold mine of your personal information. 

Take a sledge hammer and dispose in trash.