Hi jianzhang.su,
thank your for the advice.
I tested the adc_test.ino script. Connecting A3 (breakout E/1) with 2V8 (C/5) or GND has no effect.
First i tried the adc_test.ino script with the complete assembled RePhone. For a second test i connected the Xadow GSM+BLE only with the Xadow GSM Breakout. Makes no difference.
Other examples like gpio_test_for_breakout_board.ino or pwm_test.ino worked fine.