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Cisco IP Phone 7821 unkown registration status

Hi Guys,

Hope find you all well! Since I'm running out of ideas, I decide to open this new discussion. My end customer has a CUCM cluster with one Publisher and two Subscribers, version

For some reason, some IP Phones model 7821 (SIP) are not getting registered on the CUCM and I can see that they have the "Unknow" status. This is what I already tried to solve the issue:

- delete the phones from the CUCM and add them again;

- reboot the CUCM cluster

- install the last available firmware version on all nodes and restart the TFTP

- factory reset the phones

Now comes the funnest part: the end customer has a separated CUCM cluster that is running version 12.0. If I change the voice vlan (Voice VLAN here the new CUCM cluster is) on the switch ports where this phones are connected the phones are able to get register on the new CUCM.

Question: Can this be an issue related with firewall or something like that? It's strange because I'm able to ping the IP address of the affects IP Phones from all the nodes (old cluster).

Any help will be much appreciated.

Thank you!


Bruno Marques

Nithin Eluvathingal
VIP Advisor

Hi Nithin,


Thank you for your reply. We don't have any firewall between CUCM and phones. It seems that only 7821 IP Phones are not able to get registered on CUCM. I asked the end customer to unplug the network cable from one 7821 ip phone and connect another phone model like 3911 and it got registered successfully.


I don't know what I can do more ;(


Thank you!

Bruno Marques

Hi @Bruno Marques 21 


check this link below and I hope it finds useful to you.

*** Please rate helpful post. Please mark as answer if it solves your problem/query.
regards, Ritesh Desai

can you check the phone status message . it give you some information.


I hope extension  numbers has been assigned to these 7821 phones. 





===Please rate all useful posts===

Response Signature

Bruno Marques 21

Hi Guys,

Thank you all for your tips and replies. We have just found the route cause: it was a network related issue. For some reason, the traffic for the that specific voice VLAN was being blocked on the end customer firewall.

Thank you all one more time! Take care



Bruno Marques

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