It would be be illegal if the device cause "undesired" or "unintended" interference. Since interference is the point it's perfectly legal. And there really isn't anything that carriers could do about it short of amending their terms of service to prohibit it's use; they don't own the spectrum, they are licensed to operate inside it. I agree with the post below, the 1 watt or less rule applies here and you would not need a license to operate in the GSM spectrum.