How can I set up an external beacon to flash when the phone is ringing?

Traditional telephone beacons and sounders, with ring detectors, will not work on VoIP because there is no ring signal.  You will therefore need a powered beacon or sounder instead, and use the telephone's volt-free contacts to activate it.  These beacons or sounders must be provided with a separate power supply - they cannot be powered from the telephone.  Having connected an external device to an output (say Output 1), the next step is to set the output to activate it when required.

Enter the keyword "RING" for the relevant output. 
The example above shows the output set with a cadence of 10:0, meaning continuously on.  This would be suitable for a beacon, because beacons usually flash (once per second) when permanently energised.  It might not suit a sounder, however, because it would emit a continuous tone, which might not be recognisable as a phone ringing. 
For a sounder on its own, the keyword "RINGCADENCE" is a better option, causing the sounder to be energised in time with the normal phone ringer. 
For a beacon and sounder together, it is often best to use a separate output for each as shown:

In this example, Output 1 is set to activate a flashing beacon, whilst Output 2 is set to activate a sounder in sync with the cadence of the ring signal (set on the Tone settings page).   In both cases the outputs are energised when the phone is ringing with an incoming call, and de-energised when the call is answered or disconnected.

Refer to the Logic Settings page and Tone settings page in the Installation and User Guide for more details.