Retrieving a buyer's phone number.
The buyer's phone number is returned by the API under the following conditions:
If the neither of these conditions are met and you still need to get the buyer's phone number, then you can request for it via the site:
Another work around would be to require all buyers to enter their phone number when completing checkout. You can do this by setting a flag using the SetUserPreferences API call, http://developer.ebay.com/DevZone/XML/docs/Reference/eBay/SetUserPreferences.html#Request.RequiredShipPhoneNumberPreference.