If there was any question that T-Mobile is the carrier of choice for travelers.
Through the T-Mobile Tuesdays app, existing customers can receive up to 40% off Booking.com reservations. (It doesn't seem necessary to be an existing customer: Clicking through to the portal link seems to activate the discount.)
This is being done through the T-Mobile Travel website.
In a quick search for Newark, N.J. properties, discounts ranged from 13-33% off the typical rack rate. There were a few hotel properties that also didn't qualify for the discount, so mileage may vary. Regardless, it does seem like a quick way to save some money.
There doesn't seem to be a limit as to how many reservations can be made. Reservations can be made up to eleven months in advance and the offer expires November 5, 2019.
The full T&C:
Save and redeem in the T-Mobile Tuesdays app by November 2, 2021, at 4:59 a.m. ET to receive up to 40% off available accommodation bookings in the U.S. including AK, HI, Puerto Rico & U.S. Virgin Islands and bookings for international travel. Discount is applied to price of room before taxes and any fees, including additional fees collected by the property at check-in. Reservations must be made by November 2, 2021, at 4:59 a.m. ET and can only be made up to eleven months in advance of stay and are based on availability. All reservations are subject to the terms of the property and may not be able to be refunded, see individual location for details. Not valid for existing reservations, or with any other promotion, offer, discount or coupon.