Hi B
The top up rates are extortionate, so at the end of the 12 or 24 months months or if you have used all the data before then, simply through the sim away and buy a new one. Or have a new one on standby ready to Activate. I have the 3UK mobile website bookmarked in safari so I can quickly see how much data I have used.
This sim is only for data. Not calls
For me 99% of my calls are through FaceTime audio or WhatsApp audio.
If you stick this in a spare phone that’s fine you will be able to get the Internet on that spare phone but for some reason it does not allow you to use a phone as a personal hotspot. Instead you would have to buy a dedicated unlocked 4G personal hotspot. I bought mine for about £40. (HUAWEI)
I can tether my phone to this personal hotspot or if the family are with me they can all do the same. A Max of 10 Devices can tether to it.
When ever travelling to the states if you do not have a US roaming package arranged in advance you should always turn off data roaming as the prices can be extortionate. However if you decide to stick this Sim into a phone rather than a mobile hotspot then yes that specific phone will need data roaming turned on.
Just a reminder your phone or mobile hotspot need to be unlocked for this sim.
As for dual Sim phones. I do not know too much about these as I am an apple man.