As Lemon said, VOX will help, but be aware that VOX usually don't cut in immediately. Very annoying when you talk, as it cuts half the first word. So, first send a dummy signal for VOX to react on, then after, say, two seconds, start sending the real message.
Theorical distance. 5-8 km in counties but 1km in some cities and more than 500 km in clear vision between the two points with the same height.
Generally saying : the higher the frequency the longest the distance in clear conditions.
The good point here is the FM avaibility for modem communications which could be very stable.
You can put the frequency in the 8 PMR446 channels in Eu and nobody will know the power of your device.
One additional not about VOX. It does not "let go" immediately when things go quiet, so if you are going to send a response, you'll have to wait a bit there as well.
my intention is to use the example in B4A and customize it, I'm working on it, but I have little time for this, when it publishes achieve some progress here.