Every Greek number is ten digits, and a geographic one starts with 2. What follows reads left to right like an address: the second digit is one of eight regions, the third narrows it to a prefecture, and the fourth picks the exchange. So 2831 is Crete, then Rethymno, then the town itself.
How to read the map. Each patch is a prefecture — a regional unit, in the name it goes by now — and its colour is the two-digit zone it falls in, which is what the second digit of every number dialled there will be. The figure on the patch is the three-digit group serving it, and the dots are the four-digit codes, each at the town it is dialled from.
Where a patch carries two numbers. The three levels do not nest perfectly, and the map does not pretend they do. Six cities have a short code of their own and leave the rest of their prefecture on a longer one, so Larissa reads 241/249 and Heraklion 281/289. Two more prefectures share a border and a set of codes: Karditsa and Trikala, Argolis and Arcadia. Where the reverse happens and one group spans two prefectures — 223 covers Phthiotis and Evrytania, 227 covers Chios, Icaria and Samos — the patches are drawn as the single area they are.
Athens, and two codes without a dot. The 21 area is Athens, Piraeus and Elefsina, and has no four-digit codes to pin: an Athens number is 21 plus eight digits. Elefsina sits in West Attica, which is otherwise 229, and is the one place the colour is a simplification. Two codes are listed with no dot at all — 2654 Metamorfosi and 2293 Agia Sotira — because each name belongs to several villages and nothing in the sources says which one. A code without a location is honest; a dot in the wrong village is not.
Sources. Code list: Wikipedia, Telephone numbers in Greece, which groups the codes under the prefecture each serves. Code locations: the coordinates on each town's own Wikipedia article, and Wikidata for the dozen it has no article for. Geometry: Eurostat GISCO, NUTS 3 regional units at 1:1m, 2024 edition, dissolved by code group, numbering zone and administrative region.