Howdy! Welcome to the forums. Sorry it started on a bad note 
First, you might want to disconnect your system from the internet in order to review the extent of the damage and prevent further immediate exploitation. This might be a misconfigured dialplan that you can fix in the FreePBX GUI, or, it might be worse eg. an SSH access / root compromise.
Please consider reviewing some documentation on security best practices:
Then, one approach could be to spin up a new system entirely, secure it using those guidelines as a starting point, and finally restore from your backup of the hacked system and make any necessary changes to your dialplan, SSH passwords, firewall, etc., to prevent further problems.
Edited to add another good read on this:
…which links back to some forum threads: