1858 is not a country code. Numbers inside a country start like this: California (United States). The first digit 1 is the trunk prefix for domestic calls and is dropped from abroad.
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| Area or operator | From abroad | Inside the country |
|---|---|---|
 California · United States | +1 858 … | 1858 … |
| La Jolla, CA · United States | +1 858 … | 1858 … |
| Poway, CA · United States | +1 858 … | 1858 … |
| 2 more in United States | ||
| Area or operator | From abroad | Inside the country |
|---|---|---|
| California · United States | +1 858 … | 1858 … |
| La Jolla, CA · United States | +1 858 … | 1858 … |
| Poway, CA · United States | +1 858 … | 1858 … |
| 2 more in United States | ||
| Area or operator | From abroad | Inside the country |
|---|---|---|
 Market Harborough · United Kingdom | +44 1858 … | 01858 … |
If a number starts with +, the first 1–3 digits are the country code. If it starts with 0 (8 in Russia and Kazakhstan), it is a domestic number: an area or operator code follows, and from abroad the 0 or 8 is replaced with + and the country code. The same digits can mean different places, so where the call came from matters.