Apple's share of China's smartphone sales declined to 6.2 percent in the third quarter from 7.9 percent a year earlier, according to Canalys. Samsung's share expanded from 14.1 percent to 21.2 percent over the same period.
The iPhone once was so popular with Chinese gadget fans that eager buyers in Beijing waited overnight in freezing weather for the 4S model. But that excitement had faded by this September's release of the 5S. Customers said it offered too few improvements.
Samsung's advantages include being able to offer carriers a mix of phones priced as low as 1,000 yuan ($150) while Apple competes only in the highest market tier, according to Wang.
Any boost Apple gets by becoming China Mobile's new high-end phone could quickly fade, he said.
"We expect this advantage can only last three months and Samsung will bring out its next flagship model soon," said analyst James Wang of Canalys.
As for subsidies, Unicom pays 2,500 yuan ($410) of the iPhone's 5,499 yuan ($900) cost in exchange for a customer signing a two-year contract to pay a minimum of 186 yuan ($30) per month.
Analysts say China Mobile will have to match those terms to achieve significant sales.
China Mobile wants to have the world's largest 4G network. It plans to have 4G services available in 16 cities by the end of 2013 and to provide coverage for 340 cities by the end of 2014.
Seaman reported from New York City.