Talvez essas atribuições de variáveis possam ajudá-lo:
$ phonedash=123-123-1234
$ phonenodash="${phonedash//-}"
$ phone=$phonenodash
$ echo $phone
1231231234
$ echo "(${phone:0:3}) ${phone:3:3}-${phone:6:4}"
(123) 123-1234
$ new_phone=$(echo "(${phone:0:3}) ${phone:3:3}-${phone:6:4}")
$ echo $new_phone
(123) 123-1234