@Mark_BC
I think your phone sends out your location as long as it is on. I wouldn't trust airplane mode.
In most phones no signals go in our out during airplane mode, until you disable it or manually enable any of the radios (mobile network, WiFi, bluetooth).
The only way to keep it from talking to headquarters is to put it in a faraday cage.
Seems useless, no better than airplane mode... or what's one advantage?
Even then, maybe it's storing your conversations when in airplane mode then uploading them all when it gets another connection.
If you trust all the apps you have installed, and you have rooted and modded your smartphone, that would make it much more difficult for someone to be able to do that.
Maybe I'm being paranoid but I think most of us have stories about how we will be talking about something and then your phone starts sending you ads for that very thing.
I was shocked a few years ago when I first saw it happening, the brazenness of it all, that e.g. we would talk about babies (other people's babies) and then we saw the phone showing pregnancy ads, and 4-5 other similar clear-cut incidents over a short period of time. We rooted the phone and installed an Android mod. Take away Google's control over your SYSTEM directory and, unless you're using Huawei or something, or install something you shouldn't, in theory you are in control. Can't do this with Apple, sorry folks.
EDIT: Trusting all your apps and/or manually enabling specific permissions. However there's also ultrasonic cross-device tracking which doesn't require any permissions:
https://www.youtube.com/watch?v=j1FfVK6sj4I
Privacy is not easy but also not impossible!