YMMV - Targeted email - Possibly another free Tmobile line? email subject line says - Celebrate the 4th with a free line

1. For a limited time in all markets, new customers activating 3 or more voice lines and existing customers with at least 2 or more paid voice lines who activate at least one more voice line will receive one of those voice lines free (via monthly bill credits) when they choose a qualifying T-Mobile ONE, Simple Choice, Magenta, T-Mobile Essentials, or Business Unlimited​ rate plan. This offer is being marketed as "Add a line for free" and "Join with two or more lines and get one free".

2. Limitations:
• One (1) free voice line maximum per account.
• Not available for T-Mobile for Business accounts with 13+ total voice lines
• Can be combined with Carrier Freedom / BreakFree and Keep and Switch on the same line.
• Can be combined with device offers on the same line.
• Can be combined on the same account with other free line promos provided that any prior promos are compatible with the required rate plan.

2. Not available for existing customers who have fewer than 2 voice lines unless they activate at least 2 new lines during the promo period to become eligible.
• For one year after accepting this offer, customers cannot cancel any lines (all line types) that were on the account when they enrolled in the offer.
• After one year, customers must keep the 2 minimum paid voice lines AND the 1 free AAL that were activated to enroll in this offer.
• No Install cancellations for lines other than the qualifying paid AAL line and free line will not impact the free line credits.
• Device offers can be stacked with this offer.

3. • Automatic enrollment in monthly bill credits for the line with the lowest MRC activated during the promo period, up to $35 / $40 (with / without AutoPay).
• The free line bill credits start within two bill cycles from activation.
• Customers keep the credit for as long as they meet the promo requirements.
• Credit applies to the voice line charge only.
• Chargeable add-ons greater than $0 will be the customer's responsibility to pay.
https://slickdeals.net/forums/showpost.php?p=138525167&postcount=281


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From Reddit:
To find your SOC:

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Login > follow steps below > can also be found under"response" after "subscriber"
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Click the three dot menu at the top right of Chrome, select More tools, then Developer tools (CTRL-Shift-I on PC)

Go to https://my.t-mobile.com/home

Once you're at the website, go back to the developer half of the window, go to the "Network" tab. Click the Name column to sort by it, and scroll down to "subscriber." Click on this, and it should open a "Preview" tab.

Click the arrow next to "myLine" to expand, and "ratePlan" should be shown.

Credit to /u/double-xor for this tip - u/super_not_clever

Link to this Wiki