Thanks for your thorough post Marcel. I just upgraded to an iPhone 6 from a classic 3GS. On that phone, I used *61* all the time without any issues. Fast forward to the 6 and I can't get it to accept any of the codes. I saw your main post and tried turning off LTE to no avail. None of the codes work.
This was the workaround I used to enable conditional call forwarding. I took out my nano sim from the 6 and put it into the ATT sim adapter that came with my nano sim card. I then put it into my old 3GS. Voila! All codes function on the 3GS. I did the *004* code, then deactivated and reactivated to make sure it wasn't a one-time fluke. Then I popped the nano sim back into the 6 and tried calling myself with my wife's phone. It forwarded my call after several rings. I then tried all of the codes again, to no avail. I started with deactivating and got error messages, then activating. More error messages.
So is this an iOS8 issue or iPhone 6 or some other setting I'm missing? How is it that most people are able to at least get the *61* command, etc. to work? Is there something I'm missing? For now, I'm happy my workaround seems to be functioning, but there has to be a better way.
Thanks to everyone in advance!
Kevin