Reset Phone Settings
When iOS 8 came out, I had the same issue across my Fords. Even after the fix releases by Apple, I still had issues. What then seemed to be the generally recommended resolution was to reset the network settings on my phone (Settings --> General --> Reset --> Reset Network Settings) and if that didn't work, then Reset All Settings (same location). I had an iPhone 4S that had the same problems, and this ended up resolving it. Of course, I then opted for the final step in the escalation chain a couple months later...I upgraded my phone.
When iOS 8 came out, I had the same issue across my Fords. Even after the fix releases by Apple, I still had issues. What then seemed to be the generally recommended resolution was to reset the network settings on my phone (Settings --> General --> Reset --> Reset Network Settings) and if that didn't work, then Reset All Settings (same location). I had an iPhone 4S that had the same problems, and this ended up resolving it. Of course, I then opted for the final step in the escalation chain a couple months later...I upgraded my phone.