When a hello number is being dialed by a spy, it normally signifies he/she is facing danger. This term for trade craft indicates a telephone call whereby the individual who answers does not disclose who he/she is nor their location, providing just a codeword or another signal whose meaning is known by the spy.
Random illustration: The hello number is called by a spy and the individual on the other end states “there is a storm in Florida at the moment. ” When another person hears this, it might signify the climate in the state of Sunshine is horrible. However, the true meaning might be the spy requires to board a plan and instantly leave the country.