AWS allows for the dynamic resizing (enlarging) of EC2 EBS Volumes. AWS FreePBX is setup by default to support this. When your instance is started or rebooted, volumes are checked and partitions are automatically resized if necessary.
NOTE: AWS now lets you resize volumes without stopping the instance the volume is attached to. However, our automatic partition resize scripts are set to only run at boot, which means you should still stop/start or reboot your instance after any volume size change to allow the partitions to be properly resized. This also allows Linux to properly rebuild the various caches and other metadata it needs for best performance based on the size of your volumes.
More information on resizing EBS volumes can be found here: https://aws.amazon.com/blogs/aws/amazon-ebs-update-new-elastic-volumes-change-everything/
All versions of AWS FreePBX since v2.1 support resizing of the Spool volume (attached as /dev/sdb on the EC2 Console, by default). If you are looking to resize the Root volume (attached as /dev/sda1), please see this guide: https://www.thewebmachine.net/wiki/smartupgrade/increasing-the-size-of-the-root-volume