What means of communication could transmit messages when the telephone lines in the trenches were cut?

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The armies of the First World War used several means of communication: field telephones, telegraphy, optical signals, couriers, radio and carrier pigeons. The telephone was essential between posts, but its cables could be destroyed by artillery or intercepted. Under these circumstances, trained pigeons carried messages from isolated positions. Some, like Cher Ami in the U.S. Army, became famous for passing on information under fire. Pigeons therefore did not replace all communications and did not constitute the central innovation of the trenches; Above all, they formed a valuable emergency solution when the lines failed.