Cell phone signals can be jammed fairly easily. It's illegal to do-so in the states, but the devices are pretty easily obtained from what I've read. I'd imagine even easier in other countries.
A very popular Chinese online shopping site has $50 GSM/WCDMA/HSDPA/LTE jammers that fit into your pocket and they offer free shipping all over the world. I imagine that getting one sent to the US would be very easy and since only 1% of all parcels are opened for inspection it would be unlikely to be ever found out.
