Military. This is a frequency in the US Airforce maintained HF-GCS network. 4724, 6739, 8992, 11175, 13200, and 15016 are the most commonly used frequencies. It is quite common, normal even, for the same message to be sent across multiple or even all of those frequencies at the same time. If you hear phonetics on one freq try the others and see how many you hear.
There are two basic forms of voice encrypted, phonetics based, messages, "EAM"s and "Skykings". The message you heard is probably what hobbyist have come to call an "EAM". It should be noted that not all encoded traffic like that are actually EAM's, but since it is essentially impossible for a hobbyist to tell one form of message from the other they all get called "EAM". When you hear a Skyking it will be obvious.
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