The "additional" SRS radios (that go beyond what the aircraft iteself has in DCS) cannot be used to contact any ingame entity - they are only for talking to other players on SRS.
Like Daniel said, you will need to contact the AWACS on your primary radio. Most MP servers usually preset the frequencies correctly though, so one of them might be AWACS. To check this just swap through the channels and look at what frequency the SRS overlay shows for that radio.