Extension can call the other extension by calling ring group however these two extension cannot call each other only by calling ring groupHow are they dialling the ring group, dialling the RG number directly? If that is the case, then there is no way to block an extension from dialling that, other than using the sets internal dialplan. If they reach the ring group some other way, then there might be something that can be done.
If you were to changed the ring group number to something new, then created an extension that was forwarded to the new RG, you could block certain extensions from calling that extension and reaching the RG. That is, until someone discovers what the real RG number is.
What you mean is that you don't want them to be able to call each other. If that is the case, and the ring group numbers remain the same, then 3CX cannot block them.these two extension cannot call each other only by calling ring group
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