Yes! Using VoIP services for calling and instant messaging can be a great way to keep in touch rather than paying extra for traditional voice calls & SMS messaging.
Please note you will need to ensure that your device is setup correctly. You should check whichever apps you are using all utilize the data connection (VoIP/Instant Messaging) and not traditional methods (Voice calls / SMS messaging) to communicate. Should you make any traditional voice calls or send SMS messages these would be chargeable, regardless of if they were unintentional!