Grandstream PBX Troubleshooting Expert Needed

I'm seeking a skilled technician to troubleshoot issues with my Grandstream PBX system. The following features are not working as expected:

- Call Forwarding
- Voicemail
- Conference Calling

The specific problem is that these features don't activate correctly.

Ideal Skills and Experience:
- Expertise in Grandstream PBX systems
- Strong troubleshooting skills
- Experience with VoIP and telecommunication systems
- Ability to diagnose and fix activation issues

Please provide a detailed plan and previous experience.