When I was in the USA travelling and wanted to call someone on Skype back in the UK, I used the ring once hang up technique to signal for them to get logged onto Skype or text me back to say they cannot. When inthe USA on a UK mobile, it's about $1.50 per minute to make a call, and $0.50 to send a text. It's free to receive texts though so this hack let me communicate basically for free.