custom phone number format help needed

I have a column of phone numbers in this format: ########## (ie - 1234567). I need them to be in this format: ### ###-#### (ie - 123 456-7890)

I created a custom format type ### ###-#### and tried to make all the cells in that column this format but excel won't convert / take the format.

Any suggestions on how to fix or get the cells to be the format i need which again is
### ###-####
123 456-7890