Yes, If you use Excel or similar spreadsheet, this macro should do it:

 =replace(replace(XX,4,0,"-"),8,0,"-")   where XX represents the cell of the topmost phone number. The format assumes three digits for area-code followed by seven-digit number.  Then, highlight and drag down the rest of the column. If you have another spreadsheet, there is likely a similar macro to 'replace.'

I'm not using any spreadsheet software.

You might try google docs.

You need to elaborate. Use google docs for what?

I decided to export and import again after deleting the old list. After importing the phone format was ok: 123-456-7890

But, after closing thunderbird and opening it, Thunderbird remove the format: 1234567890

Don't know why Thunderbird would change this.

I found that thunderbird wasn't changing the phone number formats. Two-thirds of the phone numbers have no separators: (1234567890). I have no idea how they got that way. Now I'm trying to figure out how to fix this.

My thought was that Google Docs has a spreadsheet that might do the correction for you.

I wouldn't know where to begin with docs.

I'm getting no assistance at Google help!! I really have no clue who to ask about this!!

Well, it's not really a Thunderbird issue, so I gave a suggestion on how to fix it. But do you need the dashes? I believe Thunderbird will still store the numbers for you.

The numbers are easier for me to read. And yes, thunderbird still stores the numbers.