There is Shopify Functions, where you can customize your checkout, but this is advanced as the specialist told you 
Topic summary
Need: Require collecting a customer phone number during Shopify checkout because a supplier mandates it. Current limitation: Shopify’s native checkout settings only collect either an email or a phone number as the primary contact; the merchant seeks a way to ensure a phone number is always provided.
Recommended approach:
- Use Shopify Functions to customize checkout behavior. Shopify Functions are developer tools for extending server-side checkout logic.
- This is considered advanced and typically requires a developer to implement.
- Official developer documentation was provided via a link.
Actions/next steps:
- Find/hire a developer to implement a Shopify Functions-based customization.
- Review the Shopify Functions documentation to plan the solution.
Status: No built-in setting was identified; no code solution was shared in-thread. The discussion remains open with the actionable path being custom development via Shopify Functions.