That requirement from the offer states: "Active non-T-Mobile service required; your carrier's terms also apply." Also, "Have an eligible, unlocked phone with available eSIM and be located in the U.S." It does not mention a requirement for it being a US carrier. I suspect the goal is to entice users from other carriers to experience TM service at their current location for potential customer migration. A simple workaround would be to enroll in a cheap, US based prepaid service first to meet that requirement. However, this step might be too much trouble unless you plan on being in the US for a more than a few days.