In a wireless system in which a base station provides a time division duplex (TDD) air interface having a TDD frame configuration, a method and system for controlling whether a user equipment device (UE) operates in a first mode in which the UE sets up and engages in voice call communication via the base station or rather operates in a second mode in which the UE tunes away from the base station to set up and engage in voice call communication via another base station, such as under a different air interface protocol. The method involves determining whether traffic demand for the air interface exceeds a threshold level, and if so, causing the UE to operate in the second mode rather than in the first mode. Further, the method involves setting the threshold level based on the TDD frame configuration of the air interface.