Hey - I just found this on SubaruOutback.org. In your case apparently you don't even get a partial or temporary pairing, so the problem may be different (my money's still on the necessity to delete all previous profiles from the car AND the phone). But this would sure seem to prove that it is NOT impossible the way you were told:
Folks who are having BT problems with iOS 8 and Subaru head units - the following fix was posted on the apple support site and it worked for me....
I found a solution which worked for me with this issue.
I have a 2013 Subaru Outbackand I was having bluetooth pairing issues with my iPhone 6Plus. I could pair but the next time I started the car, it would not repair automatically. The iOS 8.1 update did not fix the problem. I had performed the pairing using the Menu buttonon my audio console. Here's the solution I found.
1. I used the Menu buttonto delete the old pairing. I turned off the car.
2. I deleted the paired connection on my iPhone. I switched off Bluetooth and restarted both the car and the iPhone.
3. I turned on Bluetooth on the iPhone.
4. This time I used voice control on the Subaru instead of the Menu button. My voice commands were Bluetooth Setup-->Phone Setup-->Pair Phone.
5. I completed the pairing on the iPhone, entering the passkey. The pairing was successful.
6. I turned off the car. A few minutes later, I turned it on and - voila - the connection was reestablished. Over the last two days, the connection has been successfully reestablished maybe nine times.
I called my Subaru service rep just to give him this info in case anyone else was having the same problem. He said that generally speaking, it's a better idea on a Subaru to use voice commands to pair. Don't ask me why. Anyway, it works.
I hope this helps others with the same problem!