[rsync]

"This is actually one of my big pain points with SIM-juggling while travelling. iMessage would work great, except for the fact that Apple unregisters your US number and registers your new SIM number in its place."

I use twilio for this. I picked a nice, easy to remember twilio number in my preferred area code and I inserted a very simple "twiml bin" that will forward that number to a number of my choosing.

So now, I can use any old SIM I like (at home or while traveling) and I just need to edit the call forward rule.

I am also able to do nice things like not have voicemail. With US carriers, you can either have voicemail, or have a confusing message about "customer has not yet set up voicemail" ... instead, my number simply rings forever...

One tip: If you want to call people with the caller-ID of your twilio number, regardless of your SIM number, you need to use a VOIP dialer (there are excellent ones) and set up a SIP endpoint at twilio (five minutes). Your outbound calls will go through the SIP endpoint and a line of code or two sets your caller ID to your twilio number.