FMT-Net = related to Have Quick military freq hopping
That's Frequency Managed Training Nets (FMT-Net). It's a term related to military Have Quick frequency hopping where a conversations "hops" quickly between many frequencies. I believe the training nets use 16 frequencies.
Frequency hopping is designed to avoid comm jamming. It is NOT encryption, but frequency hopping radios can also use encryption.
Here's a good simple description of Have Quick. If you want more search for Have Quick and FMT.