there seems no way to import phone numbers from a .csv file where the area code is separated.
That is correct. However, it should be pretty easy to merge the two columns in Microsoft excel. Here are the instructions:
Use Excel Formula
1. Insert a new column to hold the full phone number. Here we refer to this new column as Phone column
2. In the first row, enter the formula =CONCATENATE(B2, C2)
Here we assume the area code is in column B, and the local number is in column C. The above formula simply combines the area code with the local number.
3. Select the new cell, and drag it down to copy the formula for the rest of column.
Use Flash Fill
The following is copied from Microsoft support site regarding Flash Fill:
Enter the full name in cell C2, and press ENTER.
Start typing the next full name in cell C3. Excel will sense the pattern you provide, and show you a preview of the rest of the column filled in with your combined text.
To accept the preview, press ENTER.
Concatenate data with Flash Fill
If Flash Fill doesn’t generate the preview, it might not be turned on. You can go to Data > Flash Fill to run it manually. To turn Flash Fill on, go to Tools > Options > Advanced > Editing Options > check the Automatically Flash Fill box.