Apple CEO promised China Mobile subscribers "great things" after customers in Beijing asked him if the company was working on devices with bigger screens.
Cook was at China Mobile's headquarters store for the iPhone launch in China, according to Reuters.
As Cook and carrier Chairman Xi Guohua handed out autographed iPhones, customers asked questions about what the future holds for one of the most successful phones available.
"We never talk about future things," Cook said. "We have great things we are working on but we want to keep them secret. That way you will be so much happier when you see it."
Apple strived to complete the deal with China Mobile as it has over 763 million subscribers. The deal should help the smartphone company compete with Samsun and up-and-coming rival Xiaomi Tech in the future.
It took six years of negotiations with China Mobile before the two companies finally reached a deal in late 2013.
"I don't see a price war coming where Apple is engaged in the war, but I do think you're going to see a subsidy war coming," said Michael Clendenin, managing director of Shanghai-based RedTech Advisors. "China Mobile, if they're not making their targets on sales for these phones, they're going to increase the subsidies."
China Mobile's iPhones are predicted to reach 12 million during the 2014 fiscal year, according to Reuters.
China Mobile is currently charging 5,288 yuan ($870) for the basic 16GB iPhone 5S, without a subscriber contract, the same as on Apple's China website.
The carrier is also charging 4,488 yuan ($740) for a basic iPhone 5C, which is also the same price as on Apple's China website.
Reservations for iPhones reached 1.3 million as of Jan. 16, according to a China Mobile spokeswoman. Reuters research showed that a number of registrations were made using fake ID numbers however.