Who is responsible for your cellphone being hacked?

I use the hotspot on my iPhone 7 for internet access on other devices. For the past 3 weeks I've had to change my password about 3-5 times a day due to mysterious connections on my hotspot. Within a hour of password change another connection will appear. At least a dozen times I've looked at the iPhone and it says 2 devices are connected when they should only be one. When I pick up the phone and look into the camera the connections disappears. Call my phone provider the supervisor said the same thing happened to her. She said to file a police report which I did today. Phone provider says there's no fix. So I went to the Apple store talked to the techs until they closed. They said they is no fix for someone hacking into you phone. Apple tech also said hundreds for people are talking about this in the Apple forum. My phone provider and Apple say the phone is being hacked.

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Your cell phone service provider SHOULD be able to DO MORE than just tell you the phone is hacked--they can do apps or other services to return control to you.

You must be using your hotspot without a password, in which case anyone can use your phone for a connection! You also need to be aware that using a phone in this manner is already likely to be very expensive as few providers allow unlimited data over a hotspot!

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You used a public hot spot, an unsafe connection because it was cheaper.

A Wi-Fi connection only has a range of a couple of hundred metres.

If you see that effect no matter where you are, its far more likely a a bug in the firmware showing incorrect information, rather than a genuine "other" connection.

Or:
Do you have other Apple devices like an ipad or watch etc? My ipad often shows as a connection to my iphone hotspot even when it's not in use. Other devices on the same apple account seem to be able to auto connect once they have been used on the hotspot.