As noted by Nicolas Lellouche, who writes for the French tech website Numerama, Apple has updated its tech specs pages for the iPhone 18 Pro and iPhone 18 Pro Max to clarify which models are equipped with its new C2 cellular modem and where. Apple now says the iPhone 18 Pro has a C2 modem worldwide, while the iPhone 18 Pro Max uses the chip in every country except for the U.S.
Looks like it is slower and more power hungry than the C2
Article was unsure why they are doing it but they speculate that they have a contractual obligation to use some Qualcomm chips.