Unfortunately at this time we do need an email address to onboard them to the system.
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The workaround is the building admin's create an email address that they have access to, in order to add them to the system.

Then you will update their User profile by going to Users > click "edit" by their User Profile. Under "Route Intercom Calls" you change it to "Regular Phone Calls", add their phone number and then click "Save". This will route their calls to the regular phone line.