map field in user properties

hello,

Running Exchange 2013 Enterprise in hybrid mode (O365 and on premise).

We had an Active Directory field called "ipPhone" that is our internal extension.   This field is being synced to Azure AD.

We want to map this field to "Phonetic Display Name" since that field is synced between Azure AD and SharePoint Online and not currently used.

I am confused where to go to set this. I tried going to SharePoint admin center / User Profiles / Manage User Properties, then edit Phonetic Display Name.  I changed the display name to "Extension", clicked show in profile properties; but where I get down to "add new mapping" the drop down for Source Data Connection is blank.

Where should I go to map this field?

Thanks,
Claud Cutler