The format of external file needs to be in CSV (comma seperated values) format as show below
"923330000000","First Name","Last name","email@domain.com","address", "custome value 1","custome value 3","custome value 3","comments"
"923331111111","First Name","Last Name","other@domain.com","address", "custome value 1","custome value 3","custome value 3","comments"
First fields (Telephone numbers) is mandatory and other fields are optional , it means you can prepare a contact list file with only telephone numbers as show below
"923300000000"
"923331111111"
You can prepare contact list file using any text editor or preparing it in any spread sheet like microsoft Excel and then importing and save as csv format
The file extension needs to be csv like myfile.csv