Duplicate extensions do not ring both Aastra Phones

Is there some setting I am missing to allow two phones with the same extension to ring when a call comes in to said extension?

The most important thing is you can’t have two phones with the same ID’s

Can you explain? I can have two phones with the same extension which have separate MAC addresses so why won’t they both work?

Simply, because SIP does not use layer 2 (MAC addresses) it uses layer 3 (IP) two endpoints can’t have the same ip/port and be expected to work correctly.

use a ring group or ‘follow me’ instead of trying to give same extensions to a phone.

Not sure if I am explaining this correctly. I have a user who has two offices. We would like x8166 to ring at both locations. I have both phones set to be x8166 but only one of them rings. Is there a setting that I am missing or is this just not a possible feature. I am not sure how the same IP/port has anything to do with it.

Sorry if I am not understanding.

After further digging even though there is a checkbox to allow extensions to be assigned to two separate phones. The feature is not actually supported by Asterisk, so a ring group it is.

Or device and user mode.

configured on one of the programmable keys?