I know this deal has been posted multiple times before, but I wanted to share my experience with using Google voice as a temporary home for my number to essentially receive this deal without ultimately ending up with a new line. Need at least two lines on your account.
First, obviously, contact AT&T to order the iPhones you want. I bought an iPhone X and then an iPhone 8 plus as my "free phone" although your bill credits will only be for what an iPhone 8 would cost.
The iPhone X was on the line I intended to keep open (line 1). The iPhone 8 had to be opened on my new line (line 3). Immediately after receiving my phones and activating them, I ported line 2 out to Google voice for $20. This left me with two lines.
I paid off the iPhone X almost immediately which had no impact on my reimbursements. Also, I changed which phones were being used on which lines which also had no impact.
The new line phone number cannot be changed for at least 30 days or else the deal with be cancelled. Also, when a number is ported out of ATT, it remains attached to your account for 59 days and trying to port it back within that period is considered a "reclaim". This means that att can get the Google voice number as a new line, but it cannot be used to replace the line 3/temporary number.
Also, Google voice is considered a landline, and so after the port being approved, you have to call back after 3-5 days for it to finally go through.
I had my line 3 finally replaced with my original line 2 number and the credits keep coming.
It is typically free for numbers ported in to Google voice to be ported back out, but you may be charged $3.
I tried 2 times to port in my Google voice number early due to the att reps not fully understanding the rules themselves. In both cases I was able to cancel the port. I was initially worried because the unlock page on Google voice listed my number as unlocked and approved meaning they were just waiting on att to actually grab the number which would never happen. I was worded my number would never in limbo, but after discussing with Google voice support, it reverts to locked after 30 days.
It was sort of annoying to use Google voice as a primary number since texts and iMessage were kept separate and if I forgot to call someone through Google voice, they'd ask what number I was calling from. Worth it for me to save $800 though.
Let me know if you have any questions. I also welcome and fully expect the many comments about how this had been posted/isn't a good deal/everyone already knows this/etc.