Andrew...

I think the per call cost seemed high to call US compared to what I expected...maybe I was wrong...as it turned out, I really didn't make many US calls when I wasn't away from the hotel (which had wifi). But on some past trips, I had made lots of US calls, so I was a little scared of the rates.

I think my Giffgaff card charged roughly 3p/minute to call US. On arrival at Heathrow, the TFL guy made two identical charges within minutes for the two travel cards for our Oyster cards (rather than putting both on a single charge). The second charge was blocked, and I had to call my card company. The 15 minute call to the US only cost about 50p. (I think t-Mobile was significantly more than 20c/minute at the time I made my decision in April.)

Oh well...water under the bridge. However, now I have my SIMs that still have cash on them, and loading more may actually be easier than call my carrier and setting up an international plan for a week/10 days.

You have piqued my interest, however. Since our phones are unlocked and we are not on contract, I'll revisit our carrier choice and take another look at t-Mobile.

ssander