Using regex in sed to validate the length of an entry

Shell Programming and Scripting
1

I'm having trouble using sed to validate the length of an entry. I want to have a user enter a phone number of either length 7, 10 or 11. Only numbers are allowed. Does anyone know how to do this? Here's the code I have so far. It only validates that numbers are entered but not the length.

#make sure Phone Number only contains numbers
      while [[ ! -z $(echo $phone | sed 's/[0-9]//g') ]]; do
         echo ""
         printf "Please Enter The Contacts Phone Number (only numbers are allowed): "
         read phone
      done
2

try using awk..
code will be something like this..
this will check if the phone no has seven no and print the length

echo "$phone"|awk '/[0-9]{7}/{print length($0)}'