On another post I mentioned that i left my cell phone in a taxi in Budapest. No problem, my son was with me and he had his phone so just call the driver. Oooops, no number, as it was on my phone. No problem, I knew his next stop, call them. Ooops, no number, as it was on my phone.
A little googling and we got the number at the next stop and they retrieved my phone, but it was a bit unnerving for a while. I use to carry the important numbers on a small slip of paper in my wallet, but just got too casual this trip
As a backup I think its a really good idea to carry a small iPad or similar, maybe not on you, but at least back in the room Then you do have a backup form of communication. Of course, I travel alone. If yoiu travel with spouse or family, make sure everyone has all the details.