Before effecting a transaction (E100) between two servers, this method sets up (E10) a communications channel in a wireless telecommunications network between each server and a mobile terminal. One of the servers sends (E30) a signal on its wireless communications channel. That signal is received (E40) by the mobile terminal in wireless communication with the server, emitted (E50) by the loudspeaker of that mobile terminal, picked up (E60) by the microphone of the other mobile terminal, and sent (E60, E70) by wireless communication to the other server. If the received signal is validated (E90) by one or both of the two servers, the transaction is effected (E100).
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