You must have phone numbers in your google contacts group . For any question about Google Contacts group check this article: https://gsuite.google.com/learning-center/products/contacts/get-started/
Import your contacts:
Go to Contact section on Ottspott.
Click on the Add Google Account button.
Enter your Google Credentials ID if needs and click to Authorize Ottspott.
Select the right Google contact group where your contacts are.