Old IP Phones not working with Yeastar New Series PBXs


I opened a ticket about this, but I still want to get opinions from others by sharing in here. 

Some of our customers have old IP Phones (end of life) like Cisco 794x Series or Leadtek XTP8886.
These IP Phones Are working properly with Yeastar U Series PBX or 3CX IP PBX.
But can not register to Yeastar S Series or P- Series PBX.
in our tests in the lab, we see that Yeastar S/P PBX is not accepting Register request and reply with "401 unauthorized"

Do you have any idea why these phones don't work with Yeastar S/P Series while working with Yeastar U Series and 3CX?

Thanks in advance.


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