First, make sure you're getting good data from your database. It's easy to forget to select "Only Mobile Phones" in your external database filters, and end up with a list of primarily landlines.

If it's something else, here are some steps we often take to help narrow down the issue:

Count your characters. Before uploading, create a column and formula to simply count the number of characters in each phone number. For example, if your phone number is in column A, then you can create this formula in the respective rows: =len(A2), =len(A3), then you can make another column to either count, or find the unique results of that column using =unique(B:B) assuming your character counter column is column B
Bisect & Isolate. Split the list in half and upload each of those lists to prompt.io. Does one upload, and one fail? If so, split the failed one in half and upload each! Repeat this process until you find the problem. Sometimes there will be a batch of badly formatted numbers somewhere in the middle of the list, and it is easier to narrow down where in the list the bad batch is using a process like this.
Simplify The List. Similar to "Bisect & Isolate," we can isolate the problem by simplifying the list (e.g., remove some columns and all but 1 record). If that simplified list succeeds on import, keep adding records and columns back until you find the culprit!

Understanding List Import Errors

Understanding List Import Errors