Call Forwarding Breaks with iOS 8.3 Update on iPhone 6 and iPhone 6 Plus with AT&T

Question

Ever since the iOS 8.3 Update, call forwarding is no longer working on my iPhone 6 Plus.  I have spent dozens of hours troubleshooting with both AT&T Advanced Business Technical Support as well as many hours with top technicians from Apple at my local Genius Bars.
This is a critical bug for me as I used to use the call forwarding option multiple times a day, EVERY DAY, because when I am in certain business locations my mobile phone gets no reception and calls go right to voice mail.  I need to have them forwarded to the land line I'm located at. 
APPLE, PLEASE HELP!!!
I've done extensive testing, and this seems definitely to be related to Apple iOS 8.3, and it seems to be specific to the iPhone 6 and iPhone 6 Plus ONLY. 
To illustrate the EXACT problem:
A: Settings -> Phone -> Call Forwarding
B. Call Forwarding - Turn on
C. Enter Number and return to previous screen by touching the blue "Call Forwarding" to return to previous screen.
D. Call forwarding button is "green" for a split second, then returns to previous OFF state and no forwarding number is displayed.  DOES NOT WORK.
NOT AN AT&T PROBLEM.  IF SOMEONE IS EXPERIENCING THIS WITH OTHER CARRIERS PLEASE COMMENT HERE.
Further validation this is isolated to iOS 8.3 on the iPhone 6 and 6 Plus ONLY and not the sim card from AT&T...
Here are the latest steps I did with my AT&T sim card and my wife AT&T sim card (she has an iPhone 5s):
1. Removed my sim and put it in the 5s. The phone said that a "carrier update" was required and I selected "NO" to that prompt.  Call forwarding functionality worked as expected on the 5s running iOS 8.3
2. I removed the sim from the 5s and re-inserted it.  This time I selected "YES" to do the carrier update to eliminate that as a variable, and the call forwarding functionality still worked as expected.
3. I then placed my wifes sim in my 6 Plus. Call forwarding functionality does NOT work on the 6 Plus (was not prompted for carrier update). Also, keep in mind this is my second phone (Apple provided me with a new iPhone 6 Plus when they couldn't resolve the problem at the Genius Bar after hours of troubleshooting).
Note: I did one additional test which as interesting:  I put my sim in the 5s and turned call forwarding "on".  Then, I put the sim back into my 6 Plus.  The calls were still being forwarded to the number I put in while it was in the 5s, but the call forwarding interface in the 6 Plus showed that it was NOT forwarding calls.  I could not turn OFF the call forwarding from the 6 Plus and had to put the sim back into the 5s in order to turn off the call forwarding.
If anyone has an update on this, or any other suggestions please advise!  This is a critical issue for me as when I travel and when I am in poor areas of reception I need to forward calls.  This is a featured I was using multiple times a day, EVERY DAY.
APPLE?????????  HELP!!!!!!

Answer 1

Yes, I am dealing with this right now on at&t with a 6 plus. I've been down the same road as you. I got it to work with a windows phone yesterday but not my 6 plus when I switched back to the iPhone. My wifes 6 works fine. I believe it is an at&t issue and not an 8.3 issue. After dealing through several levels of support at at&t, they are telling me it has to do with their transition to hd voice. Some people are having this issue because they are updating their system and some peoples phone profile, on phones that have hd voice capabilities, are not getting added to their line and call forwarding will not work without it. I have a trouble ticket open to their engineering dept and they promise it will work for me on or before the 24th of april. Engineering has to build my phones profile so call forwarding will work with my iPhone.
This is frustrating because like you, I need to forward my calls because I have no signal in my office and need to forward to another phone so I can get my calls. If at&t would quit dragging their feet and enable wifi calling, this wouldn't be an issue for me. I would suggest you put more pressure on at&t and escalate this until it gets resolved. Or put your sim in a phone that does not have hd voice capability and see if it works then.
Good luck. I'm keeping my fingers crossed this will be fixed for me on or before the 24th. We'll see.
EDIT: I see you did put it in another phone and it worked. That backs up that the issue is at&t and their hd voice upgrades. They had no explanation as to why some work and some don't, but have them make sure a profile for your phone is on the line. If not, they'll need to build one for the phone so that it''l work. At least that's the story for now. Like I said, I'll know more in a day or two.