I often don't have a mobile or mobile number when I enter a foreign country.

We don't have the perfect solution to this yet, but we've been mulling the idea of key code locks for a while.

Current users who encounter this usually find some way to unlock their door for their guests after a phone call, leaving keys inside. The prevalence of affordable prepaid phones is helping to alleviate this issue.

I used AirBnb in Paris and my mobile didn't work. The host was gracious enough to send someone around to find the confused looking foreigners when she couldn't reach me.

It's not unusual for me to leave my phone behind when I go on vacation. That's part of the vacation.

I expect batteries to run out. You could have a usb port perhaps...