Smaller carriers, typically prepaid, are bringing the heat in the United States, especially as Sprint and T-Mobile, owners of the three largest prepaid brands in the country, look to merge.

In the case of T-Mobile’s MetroPCS, it already has plans to carry the iPhone Xs, iPhone Xs Max and iPhone Xr on the day of first retail availability — September 21 for the first pair, October 26 for the third device. Apple touts its mobile product roll-out as the fastest ever.

For US Mobile, the MVNO launched from unlocked device marketplace GSM Nation, it’s time to play ball.

The carrier has decided to offer anyone who brings in any of the new iPhone models $400 in service credit. That said, the process can get a little tricky, so if this deal’s sounding good to you, pay attention:

  1. Head to US Mobile’s site, buy a Starter Kit, which contains the SIM you need, for $3.99. Enter promo code “CHEAP400” at checkout.
  2. US Mobile will not be able to sell the iPhone Xs, iPhone Xs Max or iPhone Xr at launch, so you will have to buy one from Apple — take advantage of the iPhone Upgrade Program if need be — or another carrier.
  3. After getting their choice of iPhone, you should insert the SIM marked with “Super LTE” and then activate the device. You’ll be prompted with another promo code field — shortly after buying that SIM card, you would have receive an email from US Mobile with a second promo code. Type that in and go.

That “Super LTE” bit is important because US Mobile offers service through two sets of networks: the nation’s GSM carriers and Verizon. That Verizon service is what the company is calling “Super LTE” and that is the key to getting in on this promo. Porting your number over from another network is optional, though.

The $400 will be doled out over a year, meaning that customers’ monthly bills will be down by $33.33. Service must be continuously used for the credits to stick, so users are encouraged to enroll in AutoPay.

US Mobile offers custom-tailored plans where users dictate how many minutes, how many texts and how much data they would like to user per month at a lower price or, only on Super LTE service, unlimited talk-text-data bundles with differing speed tiers. Unlimited plans start at $35 a month and cap out at $75, minus fees and taxes.

The offer is valid through the end of October.