866 is not a country code. Numbers inside a country start like this: MTS (BARASH Communication) (Turkmenistan). The first digit 8 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 |
|---|---|---|
 MTS (BARASH Communication) · Turkmenistan | +993 66 … | 866 … |
| Area or operator | From abroad | Inside the country |
|---|---|---|
 Kabardino-Balkarian Republic · Russia | +7 866 … | 8866 … |
 Vijayawada, Andhra Pradesh · India | +91 866 … | 0866 … |
 Libonik/Vreshtaz, Korçë · Albania | +355 866 … | 0866 … |
 Ibara, Okayama · Japan | +81 866 … | 0866 … |
| Soja, Okayama · Japan | +81 866 … | 0866 … |
| Takahashi, Okayama · Japan | +81 866 … | 0866 … |
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.