It is NOT future tense.
[English doesn't have a future tense, but that's for another day]
What you mean is that English doesn't have verb forms specific to the future, but of course English does express futurity, and most commonly does so by adding "will" or "(am)(are)(is) going to". In fact "will" is most often used to express something happening in the future.
I understand that dictionaries say "will" expresses probablity, but "will" by itself is a poor probability word. Strictly speaking, the common use of "will" indicates 'definiteness'. Otherwise you need to add another word like 'possibly' or 'probably' in order to make "will" a meaningful 'probability' word.
If you are sure that is the messenger ringing, it is most logical, and easiest for English learners to understand, to say 'That is the mesenger ringing.' Otherwise, it is best to say 'That (should be)(probably is) the messenger ringing.'
It IS vitally important for ESL teachers to know what structures there are in language AND how they are used. Those who don't these things really ought to bone up.
It is also important for teachers to give a clear answer, relating to basic standard English, that will help learners grasp basic English first. Later on students can move on to the more complicated and confusing aspects of English.
"English Native Language Users/Speakers. The two dictionary entries and my posting prove that you're mistaken. The use of 'it' in such a 'will' structure would be rather strange..... not as strange as using "will"
I very explicitly warned the poster that context would be very important in deciding what to choose. I repeat again, there is no context, and one doesn't need to make up context to answer the question.
Don't presume to tell me how to answer ESLs questions, especially when you're so far off the mark. The poster asked, and I quote:
"I wondered if we could also use the other choices of 'will be' and 'has to be'. Might I ask native English teachers to help me?"
You didn't make it clear whether they could be used as the question was posed. You mainly said context. context, context.
You presume much too much, 2006, based on what? We don't know because you don't offer any proof or even the semblance of an argument.
Where is your proof? It's certainly not just referring to a dictionary definition in isolation.
You wrote; "and going into some of the illogical, although still used, things that native speakers say".
The temerity, it's truly astonishing that you suggest that you know more than AHD or M-W.
I didn't say I know more than AHD or M-W. But dictionaries often give a definition that may be sometimes used but is not necessarily the main use of a word, and may be very confusing to English learners.
I hope I have nothing more to say about this.