There are two common ways of picking up a ringing phone that's not on your desk:

• Directed Call Pickup
  • Where you pick up a particular (known) ringing extension
• General Call Pickup
  • Where you pick up any ringing phone in your pickupgroup

Default settings are:

**<exten><send>

*8<send>

For Call Pickup to work, you must modify (all) the extensions to belong to a particular callgroup and pickupgroup. For example, you might set all extensions with callgroup = 1 and pickupgroup = 1 so all extensions can pick up any ringing phone by dialing *8<send>.

In FreePBX, this is done in Applications → Extensions for each extension. Otherwise these settings are usually done in sip.conf or sip_additional.conf.