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A choke exchange is a telephone exchange designed to handle many simultaneous call attempts to telephone numbers of that exchange. Choke exchanges are typical used to service telephone phone numbers of talk radio caller and contest lines of radio stations[1] and event ticket vendors.

A central office might only have physical plant resources to handle ca. 8% of allocated telephone numbers, based on historical call traffic averages. A choke exchange has trunk facilities to other exchanges designed in a manner that high call volume is handled through the choke connection rather than overwhelming the rest of the local telephone network. Other local exchanges have a limited number of direct trunks (junctions) to the choke exchange, which may only serve one or more customers, such as a radio station contest line, which may experience many simultaneous calls. But instead of calls being overflowed to main or tandem routes shared with other calls, the unsuccessful callers receive a reorder tone from their local or tandem exchange. If the calls were overflowed to the tandem route, the caller would receive a busy tone from the exchange serving the radio station, and the sudden peak would disrupt calls between other customers.

With common-channel signaling (CCS), e.g., Signalling System No. 7, separate choke exchanges may not be required for these customers.

Examples of choke exchanges in North America have included:

Central office
202–432 Washington, DC
205–741 Birmingham, Alabama
206–421 Seattle, Washington
213–520 Los Angeles, California[2]
214–787 Dallas, Texas
215–263 Philadelphia, Pennsylvania
216–578 Cleveland, Ohio
225–499 Baton Rouge, Louisiana
305–550 Miami, Florida
312–591 Chicago, Illinois[2]
313–298 Detroit, Michigan

St. Louis, Missouri

337–920 Lafayette, Louisiana
401–224 Providence, Rhode Island
402–962 Omaha, Nebraska
404–741 Atlanta, Georgia (770 from 1995-1998)
410–481 Baltimore, Maryland
414–799 Milwaukee, Wisconsin
415–478 San Francisco, California[2]
416–870, –872 Toronto, Ontario[3]
501–433 Little Rock, Arkansas
502–571 Louisville, Kentucky
504–260 New Orleans, Louisiana
512–390 Austin, Texas
513–749 Cincinnati, Ohio
514–790 Montreal, Quebec
585–222 Rochester, New York
604–280 Vancouver, British Columbia[3]
613–750 Ottawa, Ontario
614–821 Columbus, Ohio
615–737 Nashville, Tennessee
617–931 Boston, Massachusetts
651–989 Minneapolis/St. Paul, Minnesota
704–570 Charlotte, North Carolina
713–390 Houston, Texas
714–977 Orange County, California[2]
716–644 Buffalo, New York
817–787 Fort Worth, Texas
901–535 Memphis, Tennessee
919–860 Raleigh-Durham, North Carolina
619–570 San Diego, California

One of the early choke lines (exchanges) was instituted due to a widely advertised contest by a local radio station in the Miami/Fort Lauderdale area. WHYI-FM advertised their "Last Contest". The top prize was an automobile. Since the advertising lasted over a month, there were a very large volume of calls when they announced for people to call in. There were so many calls that the local exchanges ran out of dial tones. This caused major issues since at the time if you had no dial tone, you couldn't dial at all. After it was over, the area emergency services filed complaints, and were heard. Shortly afterward the 305-550 exchange came into being. The first number on it was 305-550-9100 (for Y100 radio station). Due to the issues involved because of the "Last Contest", this may have been what caused the creation of the choke exchanges.

See alsoEdit


  1. ^ Higdon, John (1989-09-25). "Re: Prefix '520' For Los Angeles Radio Stations". Newsgroupcomp.dcom.telecom. Usenet:
  2. ^ a b c d Higdon, John (1989-09-22). "Re: Prefix '520' For Los Angeles Radio Stations". Newsgroupcomp.dcom.telecom. Usenet:
  3. ^ a b Leibold, David (1990-11-24). "TELECOM Digest Guide to Special Prefixes/Numbers". TELECOM Digest (Mailing list). Retrieved 2010-11-05.