There has never been a station officially assigned the call letters “WKRP” in the Salt Lake City area, for the simple reason that there has never been a broadcast station assigned a W— callsign in the state of Utah. (According to the U.S. Federal Communications Commission’s Call Sign Query page, the only station currently assigned WKRP is a low-power TV station in Washington, DC, WKRP-LP).
The FCC is however very lax about what stations can use as slogans. Thus, a station is allowed to refer to itself as “WKRP” hundreds of times an hour if it wants, even if that isn’t its officially assigned call. The station is also, however, supposed to announce its real callsign, followed by its community of licence, each hour near the top of the hour. In cases where stations don’t want you to know what that is, they usually will sneak in the “legal ID” announcement very quickly and quietly about 5-10 minutes before the hour, between two commercials, in the middle of a big block of commercials, possibly drowned out by sound effects, at a time when they hope people aren’t listening. (saltlakeradio.com has a good list of the radio stations in the Salt Lake City area, including their real call letters.)
BigStar303: more information than your ever wanted to know about three-letter callsigns is located at Mystique of the Three-letter Callsigns and Three-letter Roll Call.
Quasimodem: The FCC will not assign the same callsign to more than one station in the same service. But it is willing to assign the same (base) call to one each of an AM, FM, regular TV, and low-power TV station. (The FM, TV, and LPTV stations will normally have -FM, -TV and -LP or -CA suffixes in addition to the “base call”, e.g. WMUR-TV and WMUR-LP, etc.) Also, if a station is already using a particular call, other stations have to first get its permission to use the same base call.
By the way, in 1987 the FCC largely deregulated its call letter standards, and no longer reviews whether they are appropriate or not–it will assign any currently unused calls. If another station claims a new call is too close to the ones they are using, they are free to take that station to court under trademark infringement. If people think a call letter grant is obscene, they also take the station to court, and claim its call violates the “community standards” rule.