Are you sure it was for a monthly discount and not $25 debit card when upgrading?
About the T-Mobile Advantage Program
If you're enrolling in the T-Mobile Advantage Program on or after April 1, 2014, you'll receive a $25 T-Mobile Advantage Reward Card every time you purchase a new phone, device, or tablet. You can use your reward card at any T-Mobile store toward the purchase of a new phone or accessories, or apply it to your bill. For more information, read how to get your T-Mobile Advantage Reward Card.