1813 is not a country code. Numbers inside a country start like this: Brandon, FL (United States). The first digit 1 is the trunk prefix for domestic calls and is dropped from abroad.
Have the full number? Check it in the number lookup.
| Area or operator | From abroad | Inside the country |
|---|---|---|
 Brandon, FL · United States | +1 813 … | 1813 … |
| Florida · United States | +1 813 … | 1813 … |
| Land O' Lakes, FL · United States | +1 813 … | 1813 … |
| 10 more in United States | ||
| Area or operator | From abroad | Inside the country |
|---|---|---|
| Brandon, FL · United States | +1 813 … | 1813 … |
| Florida · United States | +1 813 … | 1813 … |
| Land O' Lakes, FL · United States | +1 813 … | 1813 … |
| 10 more in United States | ||
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.