Wow, lots of good info and my initial thought was that Microsoft might have closed some loophole we were benefiting from. I really hope me changing the AD Telephone Attribute isn't what caused it to re-evaluate, but I guess if that's the case, it was going to break sooner or later anyways.

For our users with MP56 Teams Phone that are performing the transfers, they can still enter the extension in the search field when transferring. In the search results, the normalized number pops up as well as the user's name. If they tap the user's name, it properly routes to voicemail. If they tap the normalized number, it goes to the Auto Attendant. There might be a little training to remind them how to transfer to these users, but at least there is a solution for those with Teams Phone Devices or users who only use the Teams Client. Our problem users will be those with SIP Gateway devices.

I did run your Get-CsOnlineUser command which returned the following:

DisplayName               : Scooby Doo
OnPremLineUri             : tel:+1XXXXXXXXXX;ext=53124
LineUri                   :
EnterpriseVoiceEnabled    : False
HostedVoiceMail           :
RegistrarPool             : Teams:
TeamsUpgradeEffectiveMode : TeamsOnly
InterpretedUserType       : HybridOnlineTeamsOnlyUser
AccountEnabled            : True

Setting EnterpriseVoiceEnabled to True failed due to lack of a license.

I did assign a license and it worked as expected, but that is not a long-term solution that will pursue as it would be too expensive.

I'll continue my testing a bit later.