UBS analyst Steve Millunovich this week bumped up his unit estimates for Apple’s iPhone on the strength of the company’s retention rates and installed base.

According to Millunovich as cited by Barron’s, “the question of iPhone growth is a matter of when, not if.”

Despite forecasting a longer upgrade cycle due to “only modest differentiation in the iPhone 7,” Millunovich raised his estimates for iPhone shipments in the September quarter to 45 million, up from his previous estimate of 40 million. Millunovich also bumped up his forecast for the December period from 70.8 million to 72 million and boosted his fiscal 2017 estimate from 207 million to 222 million units.

The latter figure would work out to growth of about 5 percent for the iPhone in 2017, Barron’s said.

While growth in 2017 is expected to remain in the single digits, Millunovich’s outlook for 2018 was a bit brighter.

The analyst said UBS is forecasting a “larger cycle” of around 264 million units in fiscal 2018, for growth of about 19 percent.

In the most recent quarter, Apple said it sold 40.4 million iPhones, a total that was down 15 percent year over year. Those slumping sales – as well as a dip in sales of iPads, Mac computers and Apple Watches – dragged the company’s quarterly revenue down 15 percent to $42.4 billion.

Despite the slack, however, Apple at the end of July celebrated the sale of its one billionth iPhone.