Just over a month since the company confirmed issues between its iPhone 4S smartphone and China Mobile SIM cards, Apple has adapted the device to eliminate signal issues, the Chinese operator has announced.

On March 1, China Mobile said that Apple has modified how the iPhone 4S interacts with the GSM chip, ensuring that after February 8, all devices would support the world’s largest network without issues.

For those that purchased an iPhone 4S before that date (and are experiencing issues), Apple is expected to roll out an iOS update soon after its iPad 3 launch event on March 7. The carrier has said it will notify iPhone 4S owners as soon as it is rolled out.

The issue is believed to have been caused by GSM chips which support the P frequency band (885-915MHZ), but not the E frequency band (880 – 890MHz). This led to many users finding they had full signal in indoor areas, but were unable to make calls.

It’s an interesting case for Apple, particularly as China Mobile is yet to become one of the company’s official Chinese partners. With 655 million subscribers, it is the world’s largest operator, and today announced that it had surpassed 15 million iPhone users in China.

Since the iPhone 4S went on sale in October, it has seen 5 million new iPhone activations, as consumers purchased the device on the black market or via overseas channels before it launched in China earlier this year.

China Mobile subscribers are forced to use the iPhone 4S on the carrier’s 2G network as Apple has, as yet, been unable to support China Mobile’s 3G (TD-SCDMA) network.

This has been the primary reason given by the operator for its position as the sole carrier not selling the iPhone, but it is a situation that China Mobile says will be remedied when it completes LTE networking testing come June.