It is fairly easy to install a new D6603 to work completly on the AT&T network. It is not so easy to find the answer of how to do it in the threads and posts. I tried many suggestions and wasted a lot of time. Hopefully this will save you from that fate.
First you must know that you can't use the IMEI number of your new phone if you want it to work completly as it should. If you use the Z3 IMEI you will have trouble with data of various sorts and the phone will go off line when on LTE after 20 or 30 minutes and you will not receive calls or SMS messages. I do not know why but it has been suggested that the AT&T network does not recognise this phone so it causes trouble.
If you have an AT&T nano-sim from your old phone then just put it in the phone. (Cut downs work but can cause other problems some say.) When the SIM is inserted for the first time into a new phone there is a prompt that asks if you want to download the APN. Hit 'yes' and the phone downloads the APN itself.
If it does not ask then go to Settings>More>Mobile Networks>Prefered Network Type and be sure LTE (Preferred) is selected. Then connect your phone to the internet and then go to Settings>More>Internet Settings and select AT&T. You should be good to go.
If you need to get a nano-sim then go to the AT&T store or call and they will give you one or send you one for free. You will need to use the IMEI number from a LTE phone that is already on the AT&T system. You can use your old phone or borrow the number from someone. If the rep installs the sim in the Z3 with the 6603 IMEI don't worry. Just call support later and tell them you sent your new phone back and want to go back to your old IMEI or have a new IMEI (from an existing AT&T phone). They will enter it and off you go. The phone from the IMEI will show in the picture on your account page. Big deal!
First you must know that you can't use the IMEI number of your new phone if you want it to work completly as it should. If you use the Z3 IMEI you will have trouble with data of various sorts and the phone will go off line when on LTE after 20 or 30 minutes and you will not receive calls or SMS messages. I do not know why but it has been suggested that the AT&T network does not recognise this phone so it causes trouble.
If you have an AT&T nano-sim from your old phone then just put it in the phone. (Cut downs work but can cause other problems some say.) When the SIM is inserted for the first time into a new phone there is a prompt that asks if you want to download the APN. Hit 'yes' and the phone downloads the APN itself.
If it does not ask then go to Settings>More>Mobile Networks>Prefered Network Type and be sure LTE (Preferred) is selected. Then connect your phone to the internet and then go to Settings>More>Internet Settings and select AT&T. You should be good to go.
If you need to get a nano-sim then go to the AT&T store or call and they will give you one or send you one for free. You will need to use the IMEI number from a LTE phone that is already on the AT&T system. You can use your old phone or borrow the number from someone. If the rep installs the sim in the Z3 with the 6603 IMEI don't worry. Just call support later and tell them you sent your new phone back and want to go back to your old IMEI or have a new IMEI (from an existing AT&T phone). They will enter it and off you go. The phone from the IMEI will show in the picture on your account page. Big deal!
xda-developers