It's depend on your calling card 
International City Codes for calling to Thailand
 
city phone
code
city phone
code
city phone
code
Ang Thong 35 Maha Sarakham 43 Ratchaburi 32
Ayuttaya 35 Mukdahan 42 Rayong 38
Bangkok 2 Nakhon Ratchasima 44 Roi Et 43
Buriram 44 Nakhon Sawan 56 Sakorn Na Khon 42
Chachoengsao 38 Nakhon Si Thammarat 75 Samut Prakan 2
Chainat 56 Nakorn Nayak 37 Samut Songkhram 34
Chaiyaphum 55 Nakorn Pathom 34 Saraburi 36
Chantaburi 39 Nan 53 Satun 74
Chiang Mai 53 Narathiwat 73 Sing Buri 36
Chiang Rai 53 Nong Khai 44 Sisaket 45
Chon Buri 38 Nontaburi 2 Songkhla 74
Chumphon 77 Pathum Thani 2 Sukothai 55
Hat Yai 74 Pattani 73 Suphanburi 35
Hua Hin 32 Phangnga 76 Surat Thani 77
Kalasin 43 Phatthalung 74 Surin 45
Kampaengphet 55 Phayao 54 Tak 55
Kanchanaburi 34 Phetchaboon 56 Trad 39
Khon Kaen 43 Phichit 56 Trang 75
Krabi 75 Phitsanuloke 55 Ubon Ratchantani 45
Lampang 54 Phrae 54 Udon Thani 42
Lampoon 53 Phuket 76 Uthai Thani 56
Loei 42 Prachinburi 37 Uttaradit 55
Lopburi 36 Prachuap Kiri Khan 32 Yala 73
Mae Hong Son 53 Ranong 77 Yosoton 45