All radio stations in the U.S. have call letters that begin with either a K or a W. Some grandfathered stations have three letters of course while others have four letters.
There are 676, (26 x 26) possible ways to combine any two letters. Since there are only two possibilities for the first letter (K or W), the number of possibilities for three letter call letters is (676 x 2), or 1352. But that forth position is tricky. It has another 26 possibilities all on it's own or (1352 x 26) = 35,152 [someone please check my math!] There are only about 14,000 stations so we have plenty of room left.
So even knowing the FCC won't let us spell out naughty words, or racial slurs etc. There are at least 15,000 remaining unused possbilities. I note this because both the first and last possibile stations have already been used. Thought only one is active.
107.5 WZZZ Portsmouth, Ohio is a small Classic Rocker serving The boonies along the West Virginia border near to Huntington, WV. But the calls are new, They flipped from WNPM on 12/16/2002 as a still new CP. Prior to that the calls lived in Syracuse on 1300 AM out of Fulton. But when Donald Derosa bought the station and weent country he dropped the oddball calls for WAMF-AM to simulcast WOLF... more radio disney.
1490 KZZZ-AM is a talker in Bullhead City, Arizona. Their ID "1490 K Triple Z" Is a one-of-a-kind branding. Wildly more exciting than KKJG "The jug", just for example. KZZZ simulcasts on that magical set of calls KAAA-AM 12230, Kingman, AZ. They also use the letter "A" there in a way that normally will get you sued.
...and if it interests you:
980 WAAA-AM was a ballsy station in Winston-Salem NC with a format aimed at the African American community. The station went on air on October 29, 1950. They dropped their long standing Black Gospel format in 2005 for Adult Standards they in turn dropped six months laterfor country oldies. In 2/1/2006, a mere four months later they went silent and returned as a simulcast of 98.3 WIST-FM. My how the mighty have fallen.