Mister in Thai is 'nai'.
Mister is said informally sometimes, depending on the informal situation but usually in tourist areas.
It is NEVER used in business, 'khun' is ALWAYS used.
I find it inappropriate, especially in a setting like a government office or hospital when they call out your name, especially if you are not a foreigner.
They shout out or speak over the speaker, Terdsak, Somchai, etc then Mister Neeranam. Next time at hospital, I'm going to ask them.