Hi Sean,We did a jabber-Iphone roll out recently and identified that the issue "A" you are having was due to the "power saving mode" on few latest IOS for us.Well it is an actual feature but cause an issue with jabber as the way it works .
You also might want to keep look out for "Wi-Fi assistant" feature if you are not using MRA. We ended up switching both the features off for the company owned devices.
Here are the links explaining the behavior
Disable Low Power Mode (Settings > Battery)
https://support.apple.com/en-us/HT205234
Turn Off Wi-Fi assist (Settings > Cellular)
https://support.apple.com/en-us/HT205296
As for your acceptable state (B) you could use the appropriate call forwarding option in the line section of the device in CUCM("Forward Unregistered" ).
***Please rate all the useful posts***
-Prabath