Country selection from entity reference

Hi,

Is there a way to automatically restrict the countries in the dropdown list with the list issued from an other entity?
I use a phone number for users which have a field "country" which references a node of the "country" content type.
The association between users and countries works fine.
I'd like to populate the phone's country code depending on value from user's country field or give the choice and restrict the proposed values to the list of existing countries (nodes from country content type).

Thanks.

Fred