It doesnt seem to be too easy, one need to have device called goTenna mesh that costs $179. The txTenna app will allows users to sync their mobile with goTenna device. Later, using UHF frequencies, allows you to transact offline and send bitcoin with cellular data.
That doesn't seem to be offline at all. You will still need data connectivity even though they advertise it being 'offline'. The only offline part I see here is the use of UHF frequencies. Afterwards, in order to completely do or execute the transaction you'll need to turn on your mobile data and that's it. There's still the need to use the internet in order to relay the tx to the nodes and the rest is history, so this gadget isn't really allowing you to send BTC in dire situations without the internet.