Voice Alert is quite simply setting your (Kenwood) APRS rig to PL tone squelch 100.0hz (for North America) (See notes for PL tone frequancies used in other countries) with the volume turned up to mid range.
This is particularly easy on the kenwood d700 rig since it can mute the speaker with the PL tone squelch while still receiving packets. Other rigs which use the 6 pin mini-din packet connector are also suitable for this mode because they allow the TNC to connect to the radio ahead of the PL tone decoder. The speaker jack of the radio is not a suitable connection for two reasons: 1) The audio will not be heard because a plug is plugged into the speaker jack, and 2) the PL tone decoder will prevent the TNC from decoding most packets.
No digipeater should ever transmit a 100.0hz PL tone!
Only stations with a live operator present should transmit a PL tone of 100.0hz
The concept is really simple. If there is an operator present at an aprs station, the radio should transmit a 100.0hz PL tone with each packet. Any other Voice Alert APRS station within simplex range will hear the packet and know that they are within range of another station. If one wishes to talk to the other, he simply calls on the APRS frequency using the PL tone. The idea here is to keep it short - just state the person's callsign and ask them to QSY to a voice frequency. Keep in mind that most digipeaters do not stand down for non packet signals, so it is likely that any voice traffic on the aprs frequency will end up doubling with other packets.
What makes Voice Alert so intriguing is that it allows a traveler to conveniently listen for other live stations. Can you imaging calling CQ CQ every minute on 146.52 while traveling? With the voice alert system, your aprs station's normal position beacon becomes a "sonar ping" to other nearby people.
It has also been suggested that if a non-standard PL tone were to be used, it would be like a private voice alert.
Non-standard PL tones have also been used on various digipeaters to verify local coverage areas. The trick here is to put a different PL tone on each of the local digipeaters and tune your PL tone squelch to one of them as you travel around town. You'll hear every packet the digipeater transmits and can assess the signal quality without having to take your eyes off the road.
No APRS Gear?
Tune your 2 meter FM transceiver to 144.39 MHz and configure encode and decode CTCSS 100 Hz. When APRS stations configured with voice alert are near, their packets will be passed to the radio's speaker, letting you know that someone is in range and probably listening. You won't know their call sign, but give a general CQ, or "calling APRS Voice Alert Stations" on the frequency to make contact, then change to another simplex frequency.
The Voice Alert PL tone use in Australia (VK) is 91.5Hz