Basically install PRTG and have it ping the IP address of your phone on your home network.
You will want to tell your home router to always assign the same IP to your phone, whenever it connects.
PRTG can call a webpage when the ping starts (or stops working) By default, it sends emails.
Now what webpage to call…I call my Ask Home Smartthings app. Since you use Ask Alexa
you could use that. I think Michaels format looks like this:
Just put in what device you want to do something with and the operator too.