R7.0 and above.
On Avaya phones with a fixed Call Log or History button (1400, 1600, 9500 and 9600 Series), if the button for CALL LOG/HISTORY or CONTACTS on the phone is selected and handset goes Off-hook then the first number on top of the list get dialed automatically.
For example, if a user is in their Call Log and a call then comes in on their handset, if they want to answer the call, they have to exit the Call Log before they can answer the call, otherwise it will make an outbound call to the Log entry they were on.
Working as designed.
The behaviour was specifically added as a feature with the Centralized Call Log and 1400, 1600, 9500, 9600, 96×1 Series handsets.
The older 4600, 2400, 5400 and 5600 Series handsets are different since they have their own Call log and functionality.
You can however raise a GRIP request with Avaya to request this functionality is removed to avoid this from happening.