this should get you started:
SELECT * FROM voice_calls INNER JOIN customers ON customers.id = voice_calls.customer_id
(note that mysql will not prefix the result column names with the table name.)

by the way, searching the mysql-docs is easy, in most cases you can just type dev.mysql.com/your_search_keyword, so
http://dev.mysql.com/join leads to
http://dev.mysql.com/doc/refman/5.0/en/join.html