Apple’s carrier partner in China, China Unicom, was hoping its deal to carry the iPhone would help it lure customers away from rival China Mobile, but many China Mobile users resisted the switch, choosing instead to modify their iPhones to work with their existing accounts. Now, the Beijing unit of China Mobile has set up a website to make it even easier.

In addition to providing instructions for how to activate an iPhone 4 with China Mobile service (changing settings to enable location-based services, for example), the website says ten China Mobile outlets around Beijing will provide a special service to help users cut their SIM cards to fit the iPhone 4’s smaller microSIM card slot.

The website offers a helpline for users with technical questions and says China Mobile Group Beijing Co. “is now working hard to make the microSIM card, and it will soon be available in our outlets.”

A China Mobile customer service representative told China Real Time the “cut your SIM card” service has been available since October 9. She confirmed that the company is working on a microSIM card and will promote it soon but there was no specific timeline.

Though iPhones have been sold in China’s thriving handset gray market for years, the devices weren’t officially launched in China by Apple until October 2009, more than two years after the device was first introduced in the U.S.

When Apple and China Unicom finally got around to launching the iPhone 3GS officially, sales got off to a slow start in part because Apple fans and early adopters already had the device and many others were reluctant to switch carriers.

To be sure, things have been changing over the past year as Apple rolls out an aggressive expansion campaign in China, including the opening of more Apple Stores and the faster release new products. The company began selling iPhone 4s in China in September–just three months after the device was launched in the U.S.–and they have been flying off the shelves. China Unicom has reported high demand for iPhones packaged with their service as well.

That rising demand, combined with a rising number of 3G users in China and reports (in Chinese) that the government may soon allow China Mobile users to keep their phone numbers when switching to China Unicom, could represent a challenge for China Mobile, which has seen user growth slow over the past year.

“It’s obvious China Mobile wants a piece of the Apple pie in China,” said Zhang Yanan, analyst for Zero2IPO, adding that “it’s very likely that China Mobile is in talks with Apple about selling China Mobile iPhones and iPads in China.”

China Mobile, the world’s largest mobile carrier by subscribers, operates a 3G network using locally-developed wireless technology called TD-SCDMA, while China Unicom operates its 3G network with the more popular WCDMA. China Unicom’s network is compatible with the iPhone while China Mobile’s is not, which is why iPhone users on its network cannot use its 3G service.

In order to offer an iPhone with 3G service for China Mobile, Apple and other handset makers would have to reengineer their devices to work on a TD-SCDMA network. A number of Apple’s rivals, including Nokia, Motorola and Research in Motion have already announced TD-SCDMA smartphones.

China Mobile has roughly 570 million users to Unicom’s 160 million, according to the figures released in September.