I had the same text problem you had with the evo 4g it's got something to do with the phone itself. Htc phones don't really utilize 3G from the airave the iPhone does as far as I can tell. My wife has an evo 4g still and she still loses 3G sometimes in the house as to where my iPhone will have it still and run faster on it then around town.
Oh and as far as I know you can only connect sprint CDMA phones to it and like 3 at a time if you shut off your phone another phone can hook up and so on its how mine works. Hope this helps
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