Old phone - locked or not?
 

Sorry if this is a stupid question. My current phone Samsung S5 is under contract with ATT. I have an old Samsung S4 laying around. Is the old S4 phone automatically unlocked since there is not under ATT contract under ATT? How can I check?

I plan to buy SIM in Hong Kong and use the S4 when I'm there. TIA.

You can check by putting in a sim card from a carrier other than the one it was used on last. If that was AT&T, then Verizon, T-Mobile or possibly Sprint. With Verizon or Sprint, the phone would have to be CDMA capable though. It doesn't matter if the sim you use for testing is active or not. It will either "No Sim" or possibly "Unable to Connect" or similar.
Alternately, you can search the web for site that will check the lock status by entering the IMEI number. This may or may not be 100% accurate, as some unlocking businesses will claim all phones as being locked to hopefully generate more business. Perhaps you could take it to a local AT&T store, and ask them to unlock it for you if it was ever used on your account. That service should be included in your plan features.

Thanks. So if it says "No Sim" that means the phone is locked, right?

not necessarily.. could be a problem with the most local towers and the bands that your phone has. I find a good way to check devices is to use the t-mobile usa site IMEI checker, where they will validate the IMEI for its contract or unlocked status. It's not 100% accurate but close enough.

If you go into a tmo store, they can put in a sim and their stores have small pico cells that broadcast basically everything their network covers, so you'll know for sure..

If you put in a non-AT&T SIM and the phone is locked, it will say something like "Insert Correct SIM". If it says "No SIM" then either the SIM card's contacts are dirty, the SIM receptacle in the phone is broken, or you didn't put the SIM all the way in.

You need to submit a request to AT&T for an unlock code. The unlock status of any AT&T phone can be checked at the same website. You do not need a sim after you receive the code from AT&T since this is a Samsung phone. This is the proces I followed for my Samsung S5.

1. Request unlock code at https://www.att.com/deviceunlock/#/
2. After receiving the unlock code: enter the following number on your dial paid (including the # sign) #7465625*638*#
3. Enter unlock code received from AT&T

The AT&T website also has the option to request an unlock code for an AT&T phone by a non AT&T customer.