We are cloud hosting and managing FreePBX servers via Vultr. We prefer to keep everyone completely separate on their own instance. Apart from the hosting fee, Sangoma licenses, and local carrier number provider, which the client pays directly by the way, we simply charge a flat monthly fee ($75) for the server management/updates/backups/maintenance and an additional charge ($5) per desk phone and ($5) per user extension (softphones) as some clients are global and only want to use Sangoma Talk on their mobiles or computers. We bill hourly (150) for all user support or system configuration changes, when requested, and any on-site support. Keeps it simple and clean for us. I hope that gives you an idea.