Bit of a strange issue, but within the FaceTime preferences on Yosemite I have the option ticked for 'Use your phones cellular connection to make and receive calls when your iPhone is nearby and on wi-fi'. When I tried to place a call it told me that the relevant setting wasn't configured on my iPhone. So, on my iPhone in Settings > FaceTime > i switched on 'iPhone Mobile Calls' however this keeps switching itself off again (tells me I need to be signed into the same iCloud account which I am) unless I also have an email address ticked as well as my phone number. So ticking an email address and then the same email address in Yosemite allow the setting to stay on and it works fine. Upon unticking the email address the 'iPhone Mobile Calls' setting turns itself back off. Its a little thing but its really annoying. Anyone know how to resolve?