|Directory||Default value||Amended value|
Put your publisher IP address into the amended service URL, click save, click update subscription & then reset your phones.
To avoid affecting other phones I did try setting these URLs in Enterprise Parameters & then setting the 7821/7841 phones to use external service provisioning, however they don't seem to support external service provisioning & thus get stuck showing "requesting". Fortunately using the amended service URLs have no negative affect on other models of phone.
Update: I also recently experienced this exact same issue with 6900 series phones after doing an upgrade to CUCM 10.5(2), fortunately updating the service URLs also fixed the problem.