The Republic of China on Taiwan currently uses the international calling code of 886 while mainland China uses 86; if Taiwan switched to the telephone numbering plan of mainland China, then its international calling code would be 86_6, one digit away from its current caling code. This is why Area 6 is reserved for Taiwan.
Codes are assigned for both the place of origin and current location of peoples who are in an area other than their permanent homeland, including travelers, temporary residents, aliens, and foreign students.
Codes are assigned for the location of individual named entities, including corporate bodies, buildings and structures, roads, waterways, railroads, monuments, camps, parks, gardens, etc. For government agencies, codes are assigned for the jurisdictions the agencies serve.
In this case, the code for the annexed entity is discontinued, and the annexed entity is assigned the code for the annexing entity.