We have CUCM v12.5 and UCCX v11.5 in our newly deployed HQ.
All phones in HQ are registering to the CUCM_Pub. 3 phones need to place in another branch which is connected to HQ via WAN (VPN) but when we take phones to the branch site they are not registering there. From the branch we can ping our HQ CUCM (Pub & Sub) and UCCX (Pub & Sub) using a PC there, but phones are not registering. All IP Phones are Cisco 7821.
I guess this might be some port issue or some Network + Security issue.
Ensure TFTP port and SIP port 5060 is open between the remote site voice VLAN and your CUCM. Make sure IP routing is working for the Voice VLAN, you can try pining the phones' IP addresses from CUCM server.