If my TGF takes her iPhone to the UK will it be possible to call her from the U.S. (without changing her SIM and calling her Thai number) ?

I'm not good with phones, thanks in advance for any advice.

Why is she going to the UK while you are in the US?

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She is helping her sister move back to Thailand.

Ok, good answer. Why don't you just call her sisters landline & problems solved or Skype her?

Where in UK is she?

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Depends on the sim card and if they have a partner carrier in the UK, but in theory it will work. It would be a better idea to just get a giff gaff sim card whilst she is over there.

I think it's a prepaid DTAC sim which needs periodic top ups.

It will be very expensive for her. She better get a UK sim card.

Does she have to do anything to make it work? Probably only necessary for emergencies.

It should do if it is unlocked.

Else she could buy cheep phone for about £20 (1,000B $30).

Advise not to buy main stream phone providers card as those are very expensive to call Thailand or the States, there are many smaller simcard providers like Giff Gaff who only charge one or two pennies a minute.

Tell her to connect phone to Internet , then use viber or Skype , it well cost her nothing , happy days

as someone already mentioned giffgaff, get her sister to order one and use that. Roaming in the UK will cost a lot, remember even if you call she pays. Or why not just use LINE or something like that, then she can log onto any Free Wifi, or even at her sisters place if she has WIFI.

www.giffgaff.com

A pre-paid sim won't have a roaming agreement in the UK and won't work here. Viber and Line are great but you assume an internet connection for them to work. Giff-gaff a good prepare in advance option, if not a Sainsbury's sim you can buy anywhere for £10 gives her a number you can call on and good for local calls.

AiS certainly have a roaming agreement in the UK (with O2). Rates are scary http://www.ais.co.th/roaming/en/pdf/Prepaid_Rates.pdf 55 Baht a minute to RECEIVE calls.

DTAC likely have a similar arrangement and fees.

Moved to communication forum.

Why is she going to the UK while you are in the US?

Nosy ain't ya, what's that got to do with anything?

If standard UK mobile charges are anything to go by, I'd say it'd be steep if it is doable. Best off just going the Skype route if she doesn't want to get hold of a local SIM.

Tell her to connect phone to Internet , then use viber or Skype , it well cost her nothing , happy days

You She will have to connect to internet via wifi otherwise you she will still pay a lot.

Does she have to do anything to make it work? Probably only necessary for emergencies.

If her iphone is unlocked, then only buy a pre paid sim card in the UK. You can phone her and it won't cost her anything.

I use a combination of GiffGaff and LINE when I am in the U.K.

A GiffGaff sim is free. Get one ordered and sent to the address ahead of her going.