Total shipments in the third quarter reached 355.2 million, up 6.8% from 332.6 million in the third quarter of 2014, IDC said.
The highest quarter ever was the fourth quarter of 2014, which hit 377.6 million units shipped, IDC analyst Ramon Llamas said. "It's interesting we haven't topped that volume yet this year," he said. "Usually the previous year's holiday volume is topped by the third quarter, but we haven't gotten there yet."
The latest numbers were actually slightly lower than what IDC had previously forecast, mainly because of lower-than-expected iPhone shipments and Android smartphone flagship models that were priced above the consumer sweet spot, IDC said.
In mature smartphone markets like the U.S., carriers are offering new services such as early trade-in options, while manufacturers are offering unlocked and off-contract deals. These factors put pressure on Android manufacturers like Samsung for phones priced above $500, said Ryan Reith, another IDC analyst.
Apple, Samsung and a number of other makers focus on the high end, with new flagship phones like the iPhone 6S and 6S Plus and the Galaxy S6 Edge Plus. But, IDC said, the bulk of volume and growth in smartphones comes from low- and mid-priced devices, especially in emerging markets in Asia and elsewhere.
Samsung was the overall leader in the third quarter, again, with 84.5 million smartphones shipped, an increase of 6% from last year. Samsung's September numbers were stronger than normal because the new iPhones weren't released until late in the month, and because Samsung launched its Note 5 and S6 Edge Plus in August.
Apple's third quarter reached 48 million iPhones shipped, up 22% from the prior year. A record-breaking 13 million iPhone 6S and 6S Plus units shipped in the first week of launch on Sept. 25.