Scott, Did you figure out the issue you were having and why the phone was rejected when you change Device Pool?  I am having the same issue on CUCM 10.5 and a 7940 ip phone.  If you could update on your resolution I'd really appreciate it. Thank you!

Hi w.phyoaung,I have the same question/problem as you but I noticed no one has offered a solution.  Were you able to find out how to disable the CBC mode cipher encryption and enable CTR or GCM? Thanks