once a call is started on your mac running Yosemite, does your iPhone go into a constant reboot loop screen?......not sure if this is isolated, downgraded to iOS 7 since it was a real bother to me so may try to reload iOS 8 again to try
The only issue I had was my phone was on the table and I placed a call from my Mac. The call had failed at some point and I didn't care enough to resume it. When I finally looked at my phone, it was still lit up saying "Call Failed"