This knowledge base article shares the proper format to use when importing phone numbers into HubSpot from Excel.
For the "Phone number" and "Mobile phone number" contact properties, the code format is as follows:+[country code][number]
If there's an extension, add the following:
+[country code][number] ext[number]
For a United States number, the Excel sheet cell will look like the below example:
+12223334444 ext123
Formatting the phone number correctly in the Excel sheet before running the import is always the best practice.