Moderator, Omni, Omni/PR, Omni/Games, FlyerTalk Posting Legend
Join Date: Oct 2004
Location: Between DCA and IAD
Programs: UA 1K MM; Hilton Diamond
Posts: 67,229
ATC just adds a phonetic letter to the end of the call sign if there are two with the same # airborne at the same time.
Had this on SEA-NRT a while back; ours went MX until the next morning, leaving about an hour before the scheduled one. IIRC, they stuck "tango" on the call sign (e.g. UA 802 Tango Heavy).