Formatting telephone numbers?

I have a set of cells that contains telephone numbers of the following types (mixed):

Cellphone numbers: 0707-905169
Small city numbers: 019-21023 or 0490-21215
Big city numbers: 08-556678 or 031-125547

How do I do to get a zero in the beginning of each number? If I use Special and "000000"-type formatting I get it right on some, but wrong on others, and as soon as a number change to a different type, it doesn´t work. I´ve also tried formatting as text input, but then references between cells and sheets fail.

If your number is in A1 in B1 following formula :="0"&a1

Brilliant. Thank you.