In this section you are able to configure the amount of time your phone will ring.
Configuration > Extensions > Click on desired Ext > Scroll down to "Inbound Dial Timeout"
Enter a numerical number in this field. The number entered is the amount of time, in seconds, the phone will ring before going to its next destination (such as voicemail).
e.g. 20 (phone will ring for 20 seconds).
Note - If you leave this field empty the default ring time of your account will be applied.