FusionPBX is a really great software. So, community is constantly expanding it possibilities and adding new features.
On of these expands - app for bulk import extensions/devices/voicemails from CSV file.
Idea is quite simple - you upload CSV file, it's being processed and shows you a preview where you can select which field corresponding to which row (or column) of the file.
Installation is quite simple. I've made it in my fork of FusionPBX, but it's fully compatible with original one.
On your server
# If you're using 4.2 of FusionPBX - replace 4.4 with 4.2 below,
# but point, only 4.4 version is supported now
git clone -b 4.4 https://github.com/samael33/fusionpbx fusionpbx-samael
mv fusionpbx-samael/app/bulk_import_extensions /var/www/fusionpbx/app
chown -R www-data:www-data /var/www/fusionpbx/app/bulk_import_extensions
rm -r fusionpbx-samael
Log into the FusionPBX webpage
Advanced -> Upgrade
Menu Defaults and Permission Defaults.
Log out and back in.