APRS is short for Automatic Packet Reporting System, and hams like to use it to transmit GPS information or to query the movement trajectory of a friend's radio. But do you know that APRS has a voice alert function?
The APRS voice alert allows the radio watcher in your mobile/portable settings to listen to the national APRS frequency, and provides a channel for all mobile APRS users to instantly call other mobile APRS users, regardless of the voice frequency that each radio is monitoring. It is like a radar detector for other mobile APRS operators within a single range on an open road. The way to do this is to lower the speaker volume on the noisy 145.175 channel and set CTCSS to 91.5Hz. This keeps your speakers quiet, but anyone who knows voice alarms knows that they can contact you by voice, simply dialing 145.175 in a 91.5Hz tone. Your APRS position transmission will be sent with 91.5Hz consonants and used as a radar ping to other voice alert radios in range.
In fact, unless another APRS voice alarm station is within the simplex range, you will never hear anything on the 145.175 voice alarm. This will ensure that you have the opportunity to make a voice call and then forward QSY to another channel for QSO. On the way, you may pass another ham, but you will never know, but if your radio has the voice alert function, you can know each other's existence, and you will have a known frequency of contact.