Contacts need to be in a CSV format. Our systems do not support other types of contact files. We recommend using Excel to save the contact file as a CSV file.
Please insure that you have the word "Phone" as name of the field that contains numbers. This is very important. Please do not use PHONE or any other variant. It is required to be Phone and specifying this differently will lead to upload errors
You should limit the number of fields in your contact file to only those that are required. Having too many fields can cause upload issues.
Additionally insure that your file does not contain any ANSI characters such as those from different languages or quotation/punctuation marks which can also be problematic.