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The one post that you linked me to has information regarding the purchase of sim cards in terminal 4. I want to top up my card, and I'll be in terminal 5.

Should I assume that the vending machines that sell the sims also dispense top up cards or vouchers and that if there is one in termial 4, there surely will be one in terminal 5. In that case, problem solved.

There's a second post in the linked thread, which I mentioned.

But why don't you just buy a SIM from eBay.co.uk and let it ship to Canada?

Regarding top ups I remember such being available in newsagencies after security.

I have never had problems purchasing topups for my UK sims from ebay.UK. And it always costs less than the face value. Been doing this for at least 5 years. So, why can't you go that route? If not suitable for you, what about topupforless, that has already been mentioned.

10 TopUp

I found a phone number and called WH Smith, a national chain of newsagents and booksellers. They have shops both air and landside in T5, and they sell SIM cards.

Or I could send one.

Thanks for all the help.

Since I already have a sim card, I don't want to buy another one. Topupforless looks as if it will do just what I need to have done, so I will likely become one of its customers, unless I find someplace in Terminal 5 that will enable me to top up, giving me a little something to do between flights.

In the US, one finds T Mobile top up cards at (what we call) drug stores. But I think I recall from past trips that the chemist Boots doesn't sell such things. I don't think I tried the bookstore.

Why T Mobile won't acknowledge foreign credit cards is beyond me! That would make life simple--but maybe less amusing.

Have you actually read in my linked postings that you can probably top up British T-Mobile SIMs with an AmEx? At least my German AmEx was accepted by T-Mobile UK, allthough my German Visas and Mastercards were refused.

That's pretty common practice with many providers in the world for the simple reason of fraud prevention. There have been many cases of fraudsters topping up prepaid SIMs with stolen credit card information in order to use the credit for calls to offshore premium numbers, which their partners ran. By accepting only local credit cards you can limit this risk significantly. Further operators are not really interested in travellers buying local SIMs. They rather want to rip you off with roaming tariffs, through which they can generate the same profit with way less usage.

i successfully toped up my tmobile uk sim with an american express prepaid card. Apparently tmobile uk accepts amex.

I've purchased Vodafone top up vouchers from the sales clerk at WH Smith. Just ask as they generate them from the register at check-out.