Newer FreePBX versions allow multiple phones to register to a single extension. See Max Contacts.

On newer FreePBX installations using chan_pjsip, you can register multiple phones to the same extension and have them all ring.

Edit the extension, on the Advanced tab, and increase the number of Max Contacts.

This is old info for older FreePBX installations running chan_sip.

http://community.freepbx.org/t/device-user-mode-feature-requests/35844

Device and User Mode supports all usage scenarios…

In D&U Mode:

• A Fixed Device with Default User is functionally identical to an Extension.
• An Adhoc Device with Default User is also functionally identical to an Extension, but with greater flexibility (another User can log into it temporarily).
• An Adhoc Device with no Default User is Registered but cannot make or receive calls - it becomes functionally identical to an Extension as soon as any User logs into it.

An Adhoc Device is “user neutral”. In other words, it has everything it needs to participate in your network except for “a User”. The act of logging in assigns that User's extension number to that device and it becomes, for all intents and purposes, an Extension. Furthermore, a User can sign in to (and/or be the default user for) any number of Devices. It's brilliantly elegant - absolutely how a modern phone system should operate.

Really the only issue with D&U Mode is that certain FreePBX modules were not written to handle it and that FreePBX does not officially support its use.