A Thai landline is nine digits with its trunk 0, and the area code is the first two or three: 02 is Bangkok and the three provinces around it, 053 Chiang Mai, 076 Phuket. There are twenty-two, each a group of provinces, and this map draws every one on the districts it serves. The colour is the digit after the 0: 2 the capital, 3 the centre and the east, 4 Isan, 5 the north, 7 the south — 6 is mobile.
How to read the map. Each patch is a code and the numeral on it is what you dial there; the city beneath it is the one the code is known by. Bangkok's 02 is the only two-digit code and takes in Nonthaburi, Pathum Thani and Samut Prakan. The north-west — Chiang Mai, Chiang Rai, Lamphun and Mae Hong Son — has two codes on the same ground, 053 and the 052 opened over it in 2014, and carries both. Four districts dial across a provincial line and are drawn where they dial: Phutthamonthon, in Nakhon Pathom, with Bangkok's 02; Nam Nao, in Phetchabun, with Khon Kaen's 043; Sam Ngam and Wachirabarami, in Phichit, with Phitsanulok's 055.
Where the lines come from. The regulator, the NBTC, assigns each code a group of provinces and publishes no map; the plan is transcribed on Wikipedia, province by province, with the four districts that leave theirs. The patches are the Department of Provincial Administration's districts, 928 of them, each drawn in its province's code unless the plan names it by itself. So the lines are provincial lines, and the four exceptions are district lines; nothing is inferred.
Sources. Codes: Wikipedia, Telephone numbers in Thailand — every area code with the provinces, and the four districts, it serves. Geometry: Department of Provincial Administration (DOPA) boundaries, districts, as published in chingchai/OpenGISData-Thailand under CC BY 4.0; dissolved by code, by region and by province.