0156 is not a country code. Numbers inside a country start like this: China Unicom (China), Honbetsu, Hokkaido (Japan), Trosa-Vagnhärad (Sweden). The first digit 0 is the trunk prefix for domestic calls and is dropped from abroad.
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| Area or operator | From abroad | Inside the country |
|---|---|---|
 China Unicom · China | +86 156 … | 0156 … |
 Honbetsu, Hokkaido · Japan | +81 156 … | 0156 … |
 Trosa-Vagnhärad · Sweden | +46 156 … | 0156 … |
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.