I come to you today with something quite weird, actually. Back from her recent trip to California, an Italian friend of mine (who speaks English at a lower-intermediate level) tells me she has been left with a rock-solid teaching about future tense in the use of American English. She seems to have experienced a clear-cut distinction between "I WILL..." and "I'M GOING TO...", by which the first ("I will") would routinely express ONLY a mere possibility, whereas the second ("I'm going to") would be used -- and REQUIRED even?!? -- to convey certainty.
So, in a phrase like "I WILL CALL YOU TOMORROW", right because the speaker is using "will", the underlying message would be that he is certainly NOT SURE he will, therefore the other person should not actually count on receiving that phone call; on the contrary, such expectation would be well founded had he said "I AM GOING TO CALL YOU TOMORROW". In other words, if it's "I'm going to call you", you can pretty much rest assured; but if it's just "I will call you", I'm not so sure myself I will (???).
I'm sort of baffled here. The fact is, in the nearly ten years that I have been using English, albeit as a foreign language, I have never heard of such a thing. I know that if I say "I am going to...", then I DO have an intention, and I'm emphasizing that. But never have I even suspected that "I will..." could be not much of a promise after all, especially not as opposed to "I'm going to...". One would think I should have noticed, but that's the downside of not being a native: you never know...
Would you please help me get this straight?
Thanks in advance.