Really depends on allot of factors, a 10dbm (which is what the board spec claims to be the limit as 10mw = 10dbm (only true for 10mw since it's a 10 x base 10 logarithmic function), but it could potentially be set to higher TX if it's only limited by software and 20-25db transmitter has a range of of 1000's of sq/m, with 20+ DBM transmit power/gain you can get into 1000M+ transmit range alone) "GSM Booster"/Picocell can easily cover an area of ~200M/sq, that's a radius of 30 ft for 1800-1900 3/4G, and more (can't recall by how much off the top of my head and I'm not doing the math) for 800-900mhz cellular signals.
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.
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...