Please help a dumb Englishman moving to America!

• Firstly, I assume an iPhone 6S Plus will work internationally on AT&T?

Yes.

• If he were to ship it to me the day after activation, would the phone be able to work in the UK, on the US SIM card, with the American number? Is there a minimum length of time a phone has to be in the US before it will work internationally?

Yes, if you purchase the international add on which is $35 for 7 days of international service. Prepaid only roams internationally in select European countries but the UK is one of them. In addition to the international add on, you must also have a regular monthly prepaid plan. There is no minimum time requirement in the US to my knowledge. 

• What information would he need to set up a prepaid account on my behalf? Would a SSN/ITIN be necessary for a prepaid account? - I do not have mine yet!

No information is required. Prepaid has no credit check.

• How long is the delay between a text being sent in the US, and receiving it in the UK whilst on AT&T?

Sorry, don’t know that.

• If something goes disastrously wrong, what is the length of time a phone would have to be outside of the US before AT&T would cut off the service?

Has nothing to do with how long it is outside the US. As long as you continue paying for prepaid service, it stays active. Once you quit paying, a prepaid account is automatically closed if the account balance is $0 for more than 60 days.

Just make sure whoever activates the SIM card for you also logs into the prepaid account and sets a password. Otherwise you might have an issue accessing the account to purchase the international add on from the UK.

@sandblaster  this is all you 

Thank you for your prompt reply @sandblaster ! May I ask if there is a pay-as-you-go option for international roaming in the UK?

If there is not, what would happen to my service if the international add-on were to lapse?

There is no pay-as-you-go option. The international add on is the only roaming option. When the add on terminates, you just won’t be able to call, text or use data unless you purchase another week.

Just to throw out numbers, the cheapest prepaid plan is $30 plus taxes. In order to use it in the UK you would have to pay $35 for 7 Days Service, that's 7 consecutive days after which you would have no service again.  

     Wouldn't it be easier and cheaper to use your current phone service and add a long-distance plan until you get to the United States and then purchase service here?   I'm not sure I understand why you need a US number before you are in the country.   

@sandblaster 

What about this, or am I not understanding something?

International PPU Calling, Messaging & Data Rates - AT&T

@lizdance40 

To call the US from my current, British phone costs £1.00 per minute, so it is more competitive than you think!

There are also considerable number of businesses and folks out there that will not accept a foreign phone number!

Those international PPU rates do not apply to prepaid. The only way prepaid can roam in the UK is with the international add-on. 

Presuming whoever is activating the prepaid plan for you is also buying that iPhone, make sure they activate WiFi calling before sending it to you. WiFi calling should work from the UK and with WiFi calling you can use the phone without paying for the international add-on. With WiFi calling, you connect to ATT’s network over WiFi so it’s as if your phone is physically in the US.

Prepaid roaming info 

https://www.att.com/support/article/wireless/KM1387490/

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