While googling for a standard that describes the complete SMS AT commands, I landed on Wikipedia's article on ITU-T V-Series Recommendations.

The keyword that I used was `itu-t v.25ter' since I came across the words twice during my attempt to locate the standard.

Well, although that standard does not say anything about SMS AT commands, I found out that it standardizes the basic AT commands that previously I knew as Hayes AT commands during my days fiddling with 56kbps modem.