"I'll phone you tonight": why is this not proper?

Question

(1) I phone her tonight.
(2) I’m phoning her tonight.
(The Cambridge Grammar of the English Language)

(1) and (2) are all possible expressions: (1) as a schedule or plan, (2) could be used in the same objects of (1), "but this is not limited to them: it could be that I have simply formed the intention to phone her (without consulting her or anyone else about the matter)”.

But for this following expression, ‘I’ll phone you tonight’, the book says “it’s hardly possible if I'd simply said, casually, I’ll phone you tonight.” What reason could be possible for this saying?

“The progressive is restricted to cases where human agency or intention is involved - hence the anomaly of examples like [ib: ‘The sun is setting at five tomorrow’]. The difference between non-progressive and progressive is fairly clear in pairs like [ii: (1) and (2) above]. The non-progressive suggests a schedule or plan: perhaps I regularly phone her on the first Sunday in the month, or perhaps the call is part of some larger plan or arrangement - it’s hardly possible if I’d simply said, casually, I’ll phone you tonight. The progressive [iib: (2) above] could be used in these schedule/plan scenarios, but it is not limited to them: it could be that I have simply formed the intention to phone her (without consulting her or anyone else about the matter) and am waiting till I think she’ll be in.” (CGEL, p.171)

Answer 1

CGEL claims that

1. the non-progressive I phone her tonight “suggests a schedule or plan” and therefore would not be employed (“it’s hardly possible”) if the speaker had only a casual intention of calling: an intention of the sort expressed by I’ll phone you tonight.

2. The progressive I’m phoning her tonight can, like the non-progressive be employed in the case of a schedule or plan; but unlike the non-progressive it can also be used of a casual intention.

I agree with CGEL, and in fact would go farther; the non-progressive does not merely “suggest a schedule or plan”, it is (in my US experience) only used of a single event when a schedule or plan is the topic.

According to my calendar she sees Bob in New York this afternoon ... I phone her tonight ... depending on what we sort out you and Carol work up the numbers overnight and email us ... then we've all got a conference call at 8:00 in the morning. 8:00 our time.

The reason, I suspect, is that a very strong non-habitual context, shared by both parties to the discourse, is required to overcome the ordinary habitual/generic implicature of the non-progressive present construction.