I'm glad I came across this thread. Since I had upgraded to iOS 8 on my 5s, I had been having problems using the Uconnect feature with my 2011 Jeep Liberty. I was able to stream music with no problem whatsoever, but when I tried to make phone calls as I had on iOS 7, I had to use the buttons on the steering wheel or radio (mine is the non NAV version-CD + Sirius model). On the phone, it would display as connected to Uconnect, but when I tried to dial calls using the Jeep's buttons, the call would connect and abruptly drop. I removed all devices from the Jeep and removed the connection from iPhone. Same result.
I then contacted Jeep and registered a complaint--to which I have yet to receive anything but an automated response. I'd encourage everyone who has issues with their vehicles to do the same after you've given the suggestions offered by the others a try.
I limped through things for a few days and then found this thread. I first tried to reset network connections, and remove the device from Uconnect. That did not work. What DID work was to reset all settings and remove the device again from Uconnect and iPhone. I'm now able to make calls and stream music again in my brief testing.
If memory serves me correct, I think I may have had to do the same thing when I upgraded to iOS 7. Thanks to those that figured this out and shared with the masses. This gives me things to consider on the next release of iOS. Hopefully those of you with problems will find resolution with the steps the others have provided and share the results.
Cheers!