CJ_London's solution will work in an update query. If there are numbers that do not start with 0, add a condition to query. Like:
WHERE Left(Telnr) = "0"
Depends on the purpose. The company I work for is implementing a SalesForce system that recognises the caller from the incoming phone number by looking it up in the customer database. They want me to provide the phone numbers exactly as the OP has specified.
Like everything, we have to decide whether to store the data in the most efficient way. Then we interface with the user in the way he wants. will you be storing the "1" for US numbers?