Account federation has been enabled between RingCentral instances in Canada, USA, Australia, and the UK. However, an issue has been identified with the Auto-Attendant functionality. When a caller provides a 4-digit extension, the Auto-Attendant is unable to route the call across instances. Interestingly, the routing works correctly when dialing by name.

From a user point of view, this works fine. I can call from the Canada instance to the USA instance via someone's internal 4-digit USA extension.

However, when an inbound call comes to the Auto-Attendant in Canada, the robot is unable to route across/between instances.

1. Call comes in from external caller to Canada (where our main numbers terminate to).
2. Auto-Attendant asks them to enter the 4-digit extension of the party they'd like to reach.
3. Caller puts in the 4-digit extension of a USA instance user (which technically does not exist in the Canada instance).
4. Auto-Attendant says an invalid selection has been made, "The extension you have entered is invalid.".
5. If the caller chooses to dial by name instead, the robot is able to route across instances from Canada to USA.

Why can the Auto-Attendant not route across instance when a caller provides a 4-digit extension, but can do so when dialing by name?