They could be pretty much ANYTHING --- either call could be a ground-command center -- an AFB's Command Post, STRATCOM Global Operations Center, USAF Global Strike Command operations center, Navy STRATCOM WING ONE Quarterdeck, Navy Submarine Squadron Quarterdeck, Navy Combined Task Force, NMCC or NMCC Site R, a ground-mobile command center, an E-6B or E-4B airborne command post, or a collective callword for a particular base/unit/group of assets.
One thing some people do is hear a callsign/callword on UHF like 311.0 or 323.8MHz, believe it to be an airborne transmitter, then they check ADS-B and correlate that call with an E-6B or E-4B that's within their UHF receiving range, but that's not often 100% accurate unless there are other indicators too.
There is a special exercise that just recently started & is lasting almost 2 more weeks.