Hi Lyssa - I don't know how to thank you enough. I have been online with Official Apple iPad Support, to Google Fi Tech Support, to the Apple store in my San Francisco Neighborhood, and finally to a 90 minute lesson at the Genius Bar. I was able to confirm that everything was working correctly -- iPad, Cellular Data SIM, Google Fi set up, etc. Along the way I met a lot of very nice, knowledgeable people, and had interesting discussions about integrating an Apple iPad into a Google Ecosystem. No one, however, could explain why I couldn't use my new iPad to send text messages to friends with Android Phones -- that is, until I received your response. Friends with iPads weren't able to help. I let the young lady from iPad Tech Support connect to my iPad and control it remotely. She wasn't able to correct the problem or even explain I was having a problem. In the last few days I have learned what a PUK Code is. My favorite response came from the young lady at Google Fi Support. She was very bright and possessed serious technical skills. In the end, however, she exclaimed in exaspertion, "I don't see why you're having problems. I have an iPhone and I don't have any problems."
Anyway thank you again,
/Charles