Critical Issue: Automatic creation of unwanted extensions from range 1000 upon mailbox creation / extension deletion

[danfw]

I am writing to report a persistent issue regarding automatic extension provisioning in our 3CX environment.

Environment Details:

• 3CX Version: v20
• Client: solave.3cx.com.mx, 72IG-XXXX-XXXX-XXXX
• Deployment: Self-hosted on AWS
• Integration: Microsoft 365 (SSO enabled)

Issue Description: Every time a new mailbox/user is created in our Microsoft 365 tenant, or whenever we delete unwanted extensions from the 3CX management console, 3CX automatically generates new extensions starting from range 1000 (e.g., 1000, 1001, etc.) and triggers the welcome email to users. We do not want or intend to automatically create extensions for these accounts.

Troubleshooting Steps Already Taken:

1. M365 Integration: Verified that "Microsoft 365 User Sync" is completely disabled. Only SSO is intended to be used.
2. PnP / Auto-Provisioning: Looked for unassigned IP phones/devices waiting in the PnP queue to ensure orphan devices are not requesting auto-assignment.
3. Behavior: Despite these checks, deleting these auto-generated extensions causes the PBX to immediately re-provision extensions from 1000 onwards and send out welcome emails.

Could you please guide us on how to trace the exact process triggering this auto-creation in the logs (Activity Log / Verbose), and advise on how to completely prevent 3CX from provisioning extensions from range 1000 automatically?

 

[TheodorosG_3CX]

Please remove your screenshot as you expose critical information to a public forum.
Also you have selected to sync all users except some specific, but you have not selected any user.

 

[Ronaldvdv]

The way I do this is go to Users -> M365. Set "Create users in a specific range starting from 9999. Make sure extention 9999 is already used (create a ringgroup or IVR with ext. 9999).
Now 3CX can not create a new user, all extentions from 9999 are used.
Create the extentions you want and make shure to add the mail adres. M365 will sync the user based on the mail address.

Other option is to create a telephony department in M365 and only sync this department. Add users you want in 3CX to this department and only those users will be synced.