The exact digits, from where you are.

From the United Kingdom, dial

What each part does

1. 00Exit code. Leaves the United Kingdom's phone network: dial 00. On a mobile, + does the same job.
2. 33Country code. Reaches France.
3. 0Leave out. The 0 at the start of the number in France is only for calls inside the country. Drop it after the country code.
4. 6 12 34 56 78The number. The rest of the number, as written in France but without the 0 at the start.