SAN FRANCISCO — Lawyers with Audet & Partners are suing Apple Inc. over high customer iPhone bills they say are caused by Apple’s failure to disclose how the phones used data.

Plaintiffs claim that they were unaware that Apple’s iOS 9 upgrade could result in their phones automatically switching from Wi-Fi to cellular data usage. The new feature, called Wi-Fi Assist, allows phones to switch to data when Wi-Fi service is unreliable. The switch led to increased phone bills for customers who unknowingly exceeded their data plan, the lawyers wrote.