Thanks to Gino for his excellent support!
After several fruitless days of emails back and forth to different OneSim support folks to get my voice calls working, it was Gino who very patiently did extensive trouble shooting and discovered the solution - now I can call out and receive calls using the 1SIM Voip app. And he clearly explained how to use either the Internet or SIM data, and how to set up my Samsung to minimize background data costs from all the other apps. Being able to speak with him on my landline while he was troubleshooting was key.