My wife and I both had iphone 5s and both were set up as mobile device presence sensors. We both just got iphone 6s. I removed the iphone 5s as devices in ST and then went through the process of adding the iphone 6s as new mobile presence devices. My iphone 6 worked no problem. My wife’s iphone 6 will not add as every time I go to add it to ST, I get the following message “Warning! This mobile phone is already setup as a mobile presence device.” However, the device does not show up anywhere in my ST. We gave the iphone 5s to our kids. I was able to add my wife’s old iphone 5 as a mobile presence device for our daughter, but when I go to add my old iphone 5, I get the same Warning message as with my wife’s new iphone 6. So, right now I have my new iphone 6 and my wife’s old iphone 5 working as mobile presence sensors, but cannot get my wife’s new iphone 6 or my old iphone 5 added because our ST thinks they are already added.
You probably have an issue with the accounts that are being used to login to the ST App. Make sure that on the old phones you logout of the original (parents) accounts. You should then be able to login on your wife’s new phone with her account. Then you can authorize new accounts for the kids phones.