When 2 VoIP phones on the same system call each other directly, it is pure data, same as sending emails or transferring files. So if you are calling phones inside your network (whether it be in the same building or not) there should not be a charge for this.
The exception to this is when you convert the VoIP to call a phone number which means it leaves your network and goes to a telephone exchange, this is where you incur costs.
This is a basic approximation of what happens.