It's included in the list of attributes synchronized by Azure AD Connect. https://learn.microsoft.com/en-us/azure/active-directory/hybrid/reference-connect-sync-attributes-synchronized
I would run through Azure AD Connect and make sure sure that attribute is selected. Confirm that you have set IpPhone as the source attribute for the primary or secondary phone in your Azure AD sync config.
If you go to Microsoft Graph Explorer, you can do a query for a user or set of users. You can specify the IpPhone attribute to confirm if it returns values for IpPhone:
https://graph.microsoft.com/v1.0/users?$select=displayName,IpPhone.