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While you are waiting for an answer.... a warning.
To ensure I am on wifi.... I turn off my data when I walk into the house or hotel.
- I have a few wifi deadzones in the house which the phone will flip over to to data when I don't want it to... especially when watching videos.
- Hotels have weak wifi when you walk away from the hallway entrance and towards the window/desk area. There's been a few media stories where travelers say they are on wifi.... when they were not.
3GB of consumption while you were disconnected from your usual points of usage is unsurprising. There are tools for your phone which allow you to monitor your usage, but would have needed to be reset at the beginning of the period. You can also review your usage patterns in the 'My Account' section of the website. My experience has been that the numbers from my phone match reasonable closely the numbers billed by Telus.
Telus measures the amount of data in and out of your phone(s) to their network by the phones unique IP address / identifier, and tallies it through your billing period.
In addition to the tips @xl mentioned, if you tethered your phone to your computer at all, you would likely use proportionally more data, as webpages fed to mobile devices are better optimized for data size compared to pages sent to a computer. Thus tethering usually uses more data.
Hope this helps!
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