How can I use the 2nd sim if my iPhone is on installment plan

You can’t pay off early and keep getting credits. When you pay it off, the credits stop. Unfortunately, the phone can’t be unlocked until paid off. Until it is unlocked, you can use the second sim if it’s an ATT sim.

That stinks especially if I agree to stay on AT&T

It's understandable how convenient a second SIM card is to customers, @jboling855!

We want to advise that @sandblaster is correct. You will need to complete installment payments in order to use a second SIM and credits stop after the device is paid in full.

Also, once you've gone through the process to unlock the device you will only be able to use an AT&T SIM (for both the physical and eSIM).

We hope this info helps. Thank you for your participation in our Community/Forums!

Lar, AT&T Community Specialist

Also, once you've gone through the process to unlock the device you will only be able to use an AT&T SIM (for both the physical and eSIM).

Ah, no, not true. I presume you meant to say “until you’ve gone through the process to unlock.”.

At this point much better to buy Apple so also you don't have commitment with ATT. It is ridiculous, I am with ATT since 1993

@Mauro555  That is true if you want to buy an unlocked phone.  However, the flip side is that you aren’t eligible for the AT&T trade in monthly credit program, which can save up to $1000 on the phone cost in the end.  You’re limited to Apple’s trade-in program, which offers a lot less, although it is a lump sum credit instead of being paid out over 36 months.  Like every choice, there are tradeoffs.

Thanks, but it is not really clear, beacuse when I put the trade-in of my 12 Pro Max, it goes down to $5.56 per month but in reality, for what you say, I pay more and they deduct $1000 at later time and monthly  but it is not clear 

The way the AT&T credit scheme works is that they give you a monthly offsetting credit on you bill over the 36 months of the installment plan.  Your bill will show a charge of the normal monthly price of the phone, which is 1/36 of the phone price.  You’ll also get a monthly offsetting credit of 1/36 of $1000 on your bill, so the effective monthly price is $5.56.  You have to keep getting billed for the entire 36 months to get the full $1000 credit.

I completely understand ATT marketing but they should do some other offers for clients like me with 30 years with ATT. I am travelling overseas and definetely I need the other country SIM, specially Italy, I can't pay all the time  $10 a day for International pass. So at this point I shoud go with Apple I am getting only $450 for trade-in but I have an unlock phone and I can pay monthly with no interests

For many years, AT&T gave special deals to people who swapped over from other carriers, and longtime customers got nothing, which obviously upset existing customers.  Now, it’s the same deals for every customer, existing or new.  Regardless, it is what it is, and anyone who wants or needs an unlocked phone shouldn’t be buying a phone from AT&T anyway.