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CP-7841: Phone Not Registered

tclsg itsupport
Level 1
Level 1

Setup: I've got a approximately 20 Cisco IP 7841 phones connected via PoE to the domain. These phones are DHCP enabled and receive their IP Address from a Windows 2016 Server. The parameters(phone number, label etc) for the phones are configured on a Cisco 3945 router. However Option 150 is on the Windows server. 

These phones have been working for the past 5 years with no problems. However on the 2nd April 2024 during working hours, all phones simultaneously stopped working with the message 'Phone Not Registered'. No change in system configuration was done. 

Initial diagnosis from the phones show that they are not receiving IP Address from the server. 

Any help in resolving this issue is highly appreciated. 


4 Replies 4

If your phones are getting DHCP from the Windows server and are no longer, I'd start there. (No

In the Status messages in the phone (under the Settings), what does the phone say about DHCP? Do you see something like "DHCP Timeout" or any other DHCP reference?

I would assume your phones are on a different VLAN than PCs, if you move a PC to the Phones VLAN is it able to get an address? What about a phone moved to the PC VLAN (even if it doesn't get the 'right' information the question would be is it getting 'any' information)?

I will assume there is a router in between the VLAN where the phones reside and the Windows server. Double-check the helper-address information just in case.

The last thing that comes to mind is the Windows server itself. Did a security update block traffic that used to be allowed? Did a certificate change that changed the way the server interacted with the network?

Start with the Status messages on the phones, though.


Thanks for your response. Going through your suggestions and will revert.

Hi Maren,

The status message on the phone reads 'Configuring IP and 'IPv4 DHCP Timeout'

There you go then! Something is blocking DHCP traffic to and/or from your phones. That sounds like a network issue to me so I would start by investigating there.