MSCs are softswitches that are currently used with the circuit-based networks that handle voice calls for 3G. They are already tied in to billing, accounting and other systems, so carriers may continue to use them instead of migrating immediately to the service management platform for pure IP networks, called IMS (IP Multimedia Subsystem), Shaw said.

Over the next few years, the mobile operators may try several different approaches to the VoIP problem. Those could include relying on terms of service that prohibit VoIP use or using network management techniques to block the third-party services, IDC's Stofega said. Alternatively, some carriers might embrace other companies' VoIP services as popular products that drive more data use.

"If they're smart, they'll figure out a way to get around this," Stofega said.