It's been 2 years..
I hope this helps someone with a similar use case to mine - using cloneable fields with text fields.
I've created a CPT with a cloneable text field to add phone numbers. And settled on using WP All Import Pro to import my CSV, instead of WP Ultimate CSV Importer (will explain why after this).
I found a snippet in WPAI's documentation to "Convert Comma-separated List into Serialized Array" (https://www.wpallimport.com/documentation/code-snippets/#comma-separated-data-to-serialized-array) and this code works too by replcing the comma with a pipe character.
So in my CSV with the column with multiple phone numbers, I separated each of it with a pipe character.
In WPAI, imported the CSV, configured my Custom Field, followed the snippet, replaced the comma with pipe character in the Function Editor, ran the import and lo-behold it worked!
Now, why didn't I use the other importing plugin?
- Sorely lacking in documentation
- Could not figure out how to map my taxonomies with sub-categories from documentation
- Could not figure out how to write the needed Header Manipulation from documentation
In conclusion, due to the lack of documentation, I just couldn't use that plugin.