Cellphone companies can’t quit Wi-Fi just yet, though not for lack of trying. Cheap and unburdened by regulations governing mobile-phone service, Wi-Fi networks have grown from a coffee-shop perk to near ubiquity. There will be more than 549 million global public and cable company-run hot spots by 2022, contributing to a technology that accounts for more than half of all internet traffic, according to equipment maker Cisco Systems Inc.

At the same time, telecom executives say fifth-generation cellular technology could drive...