In order to allow auto-answer calls, perform the following:
Log into web tools, for the extension you wish to have this function.
Go to Features > Phone Features > Ring Rules tab, and select the Create Ring Rule button.
Provide a Rule Name and set Auto Answer as the Ring Type. Save your settings.
In the new Modify Ring Rule, on the Conditions tab, select the Create Ring Rule Condition button.
To have all calls auto answered, choose the Call Type button and select All Calls.
At this point, the extension will automatically answer every call.
However, All Calls is not your only choice. You have other alternatives than can provide a more granular approach. For example, you can select Ring Hints, Caller ID, Extensions. For Call Types, besides All Calls, you chose Direct Calls, Internal Calls, External Calls, Queue Calls.