I just activated T-Mobile StraightTalk sim last night and my observation is that it usually says 3G when it's idle. It will sometimes flip to H for a moment, but then goes back to 3G which is where it stays most of the time.
But whenever I start doing anything that's data-intensive, it flips to H and says that way until I'm done doing whatever I'm doing.
It wasn't obvious this was happening until I was playing with speedtest. I'd see the phone showing 3G, then try and run speedtest to see what speed I could get at 3G. But, whenever I started the test, the phone would suddenly switch to H and stay that way for the test, then go back to 3G sometime after it ended.
I'm guessing the phone (or T-Mobile) is switching down to 3G during idleness and is only going to H when "needed" ?