“We’ve had several indications from the handset market that vendors are in real trouble,” said Tero Kuittinen, an analyst for Alekstra, a mobile diagnostics firm. “The biggest threat to all the companies seems to be the low-end Androids.”

In terms of sales, smartphones surpassed traditional flip phones this year. There are a few markets remaining where traditional cellphones are still outselling the smartphone, including India, Brazil and Russia. Data from Qualcomm suggests that Latin America, China and India are adding substantially higher numbers of smartphone subscriptions than North America, Japan, Korea and Europe.

China, with its huge population, is an attractive target for Apple. But Timothy D. Cook, Apple’s chief executive, said recently in a call with investors that the company was puzzled about why sales of its products were struggling in China. Sales there fell 4 percent in the second quarter compared with the same quarter last year. And Apple’s sales in Hong Kong were down about 20 percent.

A cheaper iPhone could help it gain traction in China, depending on its cost.

Analysts said the introduction of the cheaper iPhone would probably coincide with an expected partnership deal with China Mobile, which has about 700 million subscribers — about seven times as many as Verizon Wireless. Capturing even a small percentage of China Mobile customers would translate to tens of millions more iPhone sales.

Apple already sells its phones in China through China Telecom, a major network operator, but it slipped into sixth place among smartphone makers there in the second quarter, with a share of only 4.8 percent, according to Canalys, a research firm. Over all, China is the largest smartphone market in the world, accounting for one-third of worldwide shipments of smartphones in the second quarter; the United States is in second place, accounting for about 14 percent of shipments in the same period, according to Canalys.

Despite Apple’s efforts to keep its plans secret, clues about the new iPhones leaked out. China Telecom briefly posted a message last week on a blog platform soliciting early orders for the new devices. It identified the high-end model as the iPhone 5S, and the lower-cost one as the iPhone 5C. The post was later removed. A spokesman for China Telecom declined to comment, citing nondisclosure agreements.