When a person says "I speak English" it usually means that he can speak so called "plain" or "common" English. This is a language of literature, newspapers and "live" talks with other people. So it is a means of communication in its general meaning.
Radiotelephony is not English itself. Radiotelephony English—(RTFE) a sub-category of aviation English. Radiotelephony English is a special language used in radiotelephonycommunications between air traffic controllers and pilots. ICAO Phraseology has been developed over years and represents a very narrow and rigid subset of language. It can require the use of “general” English at times. Aviation English training should not be limited to phraseologies. Memorization of ICAO phraseologies alone does not constitute language proficiency and is an unsafe practice. Aviation English training for flight crews and air traffic controllers, then, necessarily includes practice with phraseologies but also necessarily includes a broader focus on aviation-related English.
There are a lot of words from common English but their meaning is quite different. For instance, "clear" means "allow, permit", "squawk" is not a bird's cry but it is a combination of digits used for aircraft identification, e.t.c. Besides radiotelephony contains a lot of abbreviations and special codes, which only professional users can understand (such as "wilco", QNH, QFE and so on). Grammar rules are used in radiotelephony too but in few situations.
Phraseology can be divided into two types: standard and non-standard (non-routine). The purpose of standard phraseology is to provide clear, concise and unambiguous language to pass messages of routine nature. Standardized ICAO phraseologies have been developed to cover many circumstances and mainly routine events. It is obligatory to follow these rules strictly. And the important work of developing and improving phraseologies continues. As for non-routine procedures they usually demand knowledge of common English.
According to ICAO standards a controller has to: communicate successfully either via radiotelephony or to keep up conversation with colleagues; speak on topics concerning professional problems and routine situations; use dialect or accent which is clear to other speakers.
It is necessary to underline that a controller should understand crews in every situation especially when an incident happens. A controller (and a pilot as well) should adhere to standard phrases first of all! Plain language is an option only when phraseologies are not available.
Accident investigators usually uncover a chain of events lining up in an unfortunate order to finally cause an accident, and language becomes simply one link in the chain of events. In some instances, the use (or misuse) of language has contributed directly or indirectly to an accident. The purpose of the ICAO language proficiency requirements is to avoid miscommunication, to identify and help solve a potential problem before it becomes a disaster. According to ICAO standards plain language is an option only when ICAO phraseologies are not available. The purpose of using phraseologies is to promote clarity and brevity, avoiding ambiguity.