Those SDR boards can't have that much power, can they? I mean, a usb powered radio rx/tx board running in your apartment can in no way seriously disrupt other peoples' communications.
But you don't need to be able to provide a stable connection to interfere with someone else's phone call a lucky alignment of the heavenly bodies at just the wrong moment can cause a call to drop or your phone to start roaming, deregister etc. at quite likely longer ranges than 30ft (how longer that's another question) and that can be enough you don't need to effectively build a jammer/unrouted rogue station that would constantly blanket everyone's reception in order to cause issues.
But one of the reasons I gave fairly localized examples is that while 30ft may not seem that much and it might not be, in most residential environments it's enough to potentially cause issues for some one else on the street, driving by, next door etc, and you never know what can happen.
If you want to play with GSM or any other potentially disruptive radio spectrum use it at very low power and preferably either with direct and shielded coax connections or within a faraday cage, they aren't hard to build and you can easily put both your phone and your SDR into one and work on them remotely.
P.S. Here's a good basic reference guide for RF Power/Range from digikey, it's very very basic but it's more than good enough for back of the envelope type of stuff http://www.digikey.co.uk/Web%20Export/Supplier%20Content/Lai...
I don't know what the specs for it are, but 1000mW is 20dBM which gives you a very nice coverage range on it's own and it's definitely possible to power that of USB alone.