If phone numbers import to a new, blank address, follow these steps in order until the issue is resolved:
  1. Add the correct phone address import ID (PhoneAddrImpID) to the import data file. This should be the same as the address import ID for the address to which to import the phone number.
  2. On the Fields tab, ensure the fields for each phone number have the same extension. For example, the fields for the first phone number should all have the extension 00, the fields for the second phone number should have the extension 01, etc.
If phone numbers import to correct existing address, but do not overwrite existing phone numbers, follow these steps in order until the issue is resolved:
  1. Add the correct phone address import ID (PhoneAddrImpID) to the import data file. This should be the same as the address import ID for the address to which to import the phone number.
  2. Add the correct phone import ID (PhoneImpID) to the import data file. This should be the import ID for the existing phone number.
  3. On the Fields tab, ensure the fields for each phone number have the same extension. For example, the fields for the first phone number should all have the extension 00, the fields for the second phone number should have the extension 01, etc.

For more information, refer to: