The Vista 21-IP can use either the built-in cell/IP communicator or an external device. To configure the Vista -21 for external device make sure you turn off the internal communicator using the pin switch located right below the on-board RJ-45 network port.
When the jumper is on the middle and top pin, the internal is TURN ON.
When the jumper is on the middle and bottom pin, the internal is TURNED OFF
If you leave the internal turned on, the external radio will still work, but with unstable performance. Another easy to ID symptom is when you enter programming mode and enter *29, regardless of which option you select, the next screen on the keypad will be completely blank, forcing you to do a power cycle to restore operation.
The red trouble light on the external radio will also not stop blinking if the internal communicator is turned on.