Yes, voice chat using jingle is a part of the free Spark 2.5.0+ product.
To make it work, both sender and recipient need to be running Spark, and ideally (without getting too detailed) need to be on the same LAN with no NAT or firewalls.
Spark will attempt to detect whether the recipient is capable of voice chats, and you’'ll see a ‘‘dial’’ option in the menu when you right click someone in the roster.