The "Doctor Channel" was a channel created by the Unified Intelligence Taskforce as a connection to the Doctor, giving him a contact line directly to HQ. Kate Stewart, leader of UNIT, doubted that he even remembered it, due to him never using it.
During a crisis of "frozen" aeroplanes in the air, Missy contacted UNIT on the channel via her device with a text that Mike decrypted. Missy used it to tell them she wasn't dead and they should meet her with armed soldiers. (TV: The Magician's Apprentice)