A field testing system is provided that uses a personal computer tethered to an interface board. A cellular telephone plugs into the interface board during wireless field testing. The cellular telephone may include configurable multiplexer circuitry and power supply circuitry. During normal operation, the cellular telephone is configured so that its application processor is linked to its application processor and to an external bus. During field testing, the cellular telephone is configured to link the baseband unit to the external bus and the interface board. The baseband unit may support advanced communications busses (e.g., USB). To avoid consuming too many pins in the external bus between the interface board and the cellular telephone, power for the USB bus during field testing may be derived from a power management unit in the cellular telephone.