From the release:
Free Line: Qual’g credit req’d. Customers cancelling a line after 1/1/17 not eligible. May take up to 2 bill cycles. 1/acct; must keep existing lines. Taxes/fees may be applied to pre- bill credit price on some legacy plans.
1. Qual'g credit req'd. Nice way to hide that one T-Mobile. So qualifying credit required - so is someone with less-than-stellar credit not able to get a free line?
2. Customers cancelling a line after 1/1/17 not eligible. In other words, if you already have all the lines you need, don't think you can just cancel one and then get it back for free. And if you cancel any line, you are no longer eligible to get a free line and will lose any "free" line you currently have.
3. May take up to 2 bill cycles. Why? Do humans have to process the bill credit? (yeah, right)
4. 1/acct;... So whether you currently have 2 lines or 8 lines, you can still only get 1 for free. Whoopie! (not)
5. ...must keep existing lines. There it is. Refer to #2.
6. Taxes/fees may be applied to pre- bill credit price on some legacy plans. Ah, so you'll still pay taxes on that line unless, presumably, you're on an "All-In" plan.
So basically, nobody currently on T-Mobile benefits from this except for those who may already be looking at adding a line to their account. And for those that do take advantage of this, you are then stuck with every line you have if you want to keep getting that credit.