Answer
This older Customer-specific communication-method API package was replaced by the generic Comm_Method_API package in later releases — call Comm_Method_API.Get_Default_Value instead, passing the relevant entity type, key column name, and communication-method type as parameters, for example Comm_Method_API.Get_Default_Value('CUSTOMER', 'CUSTOMER_NO', 'PHONE') to retrieve a given customer's default phone number, substituting in the actual customer number. This same generic function works the same way for other entity types and communication-method types, rather than needing a separate dedicated API package per entity type the way the older Apps8-era API worked.