Cisco 7821 - Phone not Registered - CUCM 10.5.1

[AmmarMalhotra8566]

Hello everyone, 

We bought 35 new 7821 - they are not getting registered. 

they where on 12.0 firmware i upgraded them from CUCM (10.5.1) to 12.5 added extension manually as they are sip still cannot register. 

i have to add VLAN manually in the phones for them to take voice vlan and get rid of activation code and domain. 

We have all others phones 7941 and 7942. 130 phones working. 

Need HELP. 

[AmmarMalhotra8566]

i changed the hostname to IP address fixed the issue, phone will register i know not the right way but got the job done. once i contact our Cisco account manager for certificate i will do the FQDN and DNS.