**NOTE THIS IS ILLEGAL IN THE UNITED STATES** I am not endorsing this in any way, but they do sell cellphone jammers that create enough interference to take out a small room and they are cheap, like $27.08 in some places. They really do work but it violates FCC regulations. I'm not going to link but it's only a couple links down on a search engine. Your average cell phone isn't nearly powerful enough to create interference and they are designed that way. If you can find a high-powered amplifier for an external antenna, supposing your phone has a port, many older ones do. you might be able to rig something up. A home manufactured device would probably be a bit bulky to discretely carry around though.