The exact digits, from where you are.

From the United States, dial

What each part does

1. 011Exit code. Leaves the United States's phone network: dial 011. On a mobile, + does the same job.
2. 44Country code. Reaches the United Kingdom.
3. 0Leave out. The 0 at the start of the number in the United Kingdom is only for calls inside the country. Drop it after the country code.
4. 20 7946 0958The number. The rest of the number, as written in the United Kingdom but without the 0 at the start.