Hi xloveusa,
Thank you for your question.
We could refer to the following steps to achieve it:
1.
Create a custom attribute for “Ext” prefix contact
2.
Modify the existing GAL to exclude the new custom attribute which has been created in step 1
3.
Create a new address book with new custom attribute which has been created in steps 1
We could refer to the following link to learn more about address list:
https://technet.microsoft.com/en-us/library/aa997686(v=exchg.141).aspx
Best Regard,
Jim Xu
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Jim Xu
TechNet Community Support
