Or go to Settings >> Messages >> Send as SMS and enable on the iPhone side - which will send an SMS when iMessgae is not available. Annoying, but works for the iPhone sender.
I figured this would be one of the issues (since it is off by default) but I wasn't about to send a text to all my iPhone friends to tell them to turn this setting on. Also, I have this setting turned on on my wife's iPhone but still would miss messages from her