JERUSALEM, Dec 2 (Reuters) - Cellcom, Israel's largest mobile operator, said on Thursday a 12-hour outage in its network that prevented millions of customers from receiving or making calls would hurt fourth-quarter earnings. Cellcom had 3.38 million customers at the end of September, and, as a result of its worst ever malfunction in the "core of its network", most of them were unable to make or receive calls from 0800 GMT on Wednesday.