Changing 3G Bandwidth Capacity on Devices

I had a hypothetical questions. I could be way off on this. But the sole reason a 3g phone on AT&T such as the iphone won't work on a T-mobile network is because of the bandwidth each network operates on. AT&T's 3G network operates on 2100-1900MHZ and I think T-mobile operates on 2100-1700MHZ. So assuming we could shorten the bandwidth on the device, then it would be possible to use an iphone 3g on t-mobile's 3g network.

I don't know how difficult it would be to do this. Is there a way to write some code on the apple os that would limit the bandwidth, essentially all t-mobile phones should work on at&t's network because they are within range? Or are we looking at changing some hardware? I'm thinking it might be easier to do with android phones. I dunno. But isn't this all simple electronics. A short wave radio basically can get higher range of frequencies. Our normal radios are limited. There should be a way to limit the bandwidth.