iOS/Android: It happens to the best of us: you forget your phone when you go out, or it runs out of juice. Hotel My Phone lets you use a friend's phone, but still use your phone number.
Both phones need to have Hotel My Phone installed in advance, so it isn't perfect in every situation, of course. It's unlikely you'll randomly find a stranger willing to install this app to help you out. It's good for you and your spouse, or when you have a work and a personal phone. When you're in a bind, you can switch numbers without having to switch phones. I tried this with a spare pay-as-you-go phone and it worked great when my iPhone was being repaired.
Calls within your "friend network"—people you've pre-authorized—are free. Otherwise it's $1.00 every time you use someone else's phone.