Analog devices use a flash hook signal during a call to indicate when they need additional services from the telephone system. For example, to put a call on hold or to transfer a call.

A flash hook signal is a short interruption in the call connection. Originally, analog telephone users would send a flash hook signal by briefly tapping the hook switch on their phone. On most modern phones, user can send a flash hook signal by pressing the key marked R, Recall, H, or Hold. The marking varies between different makes and models of phone.

The minimum and maximum interruption time for a hook flash signal vary between countries. For details of the different maximum and minimum width values used in various countries, refer to the Avaya IP Office Locale Settings manual.

For IP Office systems, you can adjust the settings to match local analog phones as follows: