Yup, got caught by this a few weeks ago. Of more specifically, I got caught by the fact that my dual-sim phone only supports 2G on whichever SIM you're not using for data. As I was using my AT&T SIM for data (yay to an unlimited global data plan!), that meant that the Three SIM didn't work at all in that phone.

I'd never noticed this before as obviously the other places I'd used this phone with a local SIM all had 2G support, and given that I only use the local SIM for voice/SMS, it's always worked perfectly before.

Thankfully I had another small phone that I could use instead, but I did waste an hour or so trying to work out what was going on before I realized the limitation of my phone