Its designed to drop down to GSM 3G/4G when on call. LTE chip shuts off when on call but you should still be able to pull data.
I also heard AT&T throttled while on a call so you will only get about 1mbps while on the phone.
It's dropping the 3G/4G connection about 20 seconds after the switch from LTE.
That's weird.
It shouldnt do that.
Try a reset network settings.
Also call up AT&T to see if they need to configure something on their end if the reset doesnt fix it.
I used an nano adapter and put the the SIM in my old 4S and I was able to have a call and use cellular data at the same time. Either the hardware is bad on the 5 or it is a software issue handling the switching. I am on 6.0.1.
I'd try a restore and setup as new.
Also I would give AT&T a call to make sure you're using the correct settings on their end in their system.
This phone was setup as a new phone on Monday. I never restored from a backup when I got it 4 days ago.
I would assume the settings are correct. The SIM worked in the 4S without any issues.
4S also dosent have LTE. Call AT&T and they should be able to fix your settings.
Told you to call them.
The 4S and i5 is not the same thing.
They need to adjust it on their end for the correct device on your line.
Why isn't this an automatic provisioning when the phone is activated? Even the advance tech said he didn't see anything wrong and basically said he was going to try "push" my number on the network to see if that made a difference.
Because the iphone 5 needs its own special LTE data plan in order to receive and use every feature it was intended for.
So just activating the phone with a sim doesnt provision or change the data plan on your account.
They will need your new IMEI to do that on the carriers end.