Thank you. Negative, that is not a feature that we provide at the phone level. It'd be simpler to dial in at the Asterisk level and perform a ChanSpy on the party to which you wanted to listen.
Because a phone can only be on one active call at a time though (excepting 3-way local conference calling), you couldn't have someone walk up and pick up a handset and listen in to a conversation that an agent was having over a headset. Rather, you'd do that remotely using ChanSpy from another device, to Spy on the target device's audio channel inside of Asterisk.
Digium, Inc. | Senior Product Manager